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BACKGROUND: Mirabegron is a β3-adrenergic receptor (β3-AR) agonist approved only for the treatment of overactive bladder. Encouraging preclinical results suggest that β3-AR agonists could also improve obesity-related metabolic disease by increasing brown adipose tissue (BAT) thermogenesis, white adipose tissue (WAT) lipolysis, and insulin sensitivity. METHODS: We treated 14 healthy women of diverse ethnicities (27.5 ± 1.1 years of age, BMI of 25.4 ± 1.2 kg/m(2)) with 100 mg mirabegron (Myrbetriq extended-release tablet, Astellas Pharma) for 4 weeks in an open-label study. The primary endpoint was the change in BAT metabolic activity as measured by [(18)F]-2-fluoro-d-2-deoxy-d-glucose ((18)F-FDG) PET/CT. Secondary endpoints included resting energy expenditure (REE), plasma metabolites, and glucose and insulin metabolism as assessed by a frequently sampled intravenous glucose tolerance test. RESULTS: Chronic mirabegron therapy increased BAT metabolic activity. Whole-body REE was higher, without changes in body weight or composition. Additionally, there were elevations in plasma levels of the beneficial lipoprotein biomarkers HDL and ApoA1, as well as total bile acids. Adiponectin, a WAT-derived hormone that has antidiabetic and antiinflammatory capabilities, increased with acute treatment and was 35% higher upon completion of the study. Finally, an intravenous glucose tolerance test revealed higher insulin sensitivity, glucose effectiveness, and insulin secretion. CONCLUSION: These findings indicate that human BAT metabolic activity can be increased after chronic pharmacological stimulation with mirabegron and support the investigation of β3-AR agonists as a treatment for metabolic disease. TRIAL REGISTRATION: Clinicaltrials.gov NCT03049462. FUNDING: This work was supported by grants from the Intramural Research Program of the NIDDK, NIH (DK075112, DK075116, DK071013, and DK071014).

The current obesity pandemic results from a physiological imbalance in which energy intake chronically exceeds energy expenditure (EE), and prevention and treatment strategies remain generally ineffective. Approaches designed to increase EE have been informed by decades of experiments in rodent models designed to stimulate adaptive thermogenesis, a long-term increase in metabolism, primarily induced by chronic cold exposure. At the cellular level, thermogenesis is achieved through increased rates of futile cycling, which are observed in several systems, most notably the regulated uncoupling of oxidative phosphorylation from ATP generation by uncoupling protein 1, a tissue-specific protein present in mitochondria of brown adipose tissue (BAT). Physiological activation of BAT and other organ thermogenesis occurs through β-adrenergic receptors (AR), and considerable effort over the past 5 decades has been directed toward developing AR agonists capable of safely achieving a net negative energy balance while avoiding unwanted cardiovascular side effects. Recent discoveries of other BAT futile cycles based on creatine and succinate have provided additional targets. Complicating the current and developing pharmacological-, cold-, and exercise-based methods to increase EE is the emerging evidence for strong physiological drives toward restoring lost weight over the long term. Future studies will need to address technical challenges such as how to accurately measure individual tissue thermogenesis in humans; how to safely activate BAT and other organ thermogenesis; and how to sustain a negative energy balance over many years of treatment.

Background: L-triiodothyronine (LT3) is a substitute for levothyroxine (LT4) for thyroid cancer (TC) patients during the preparation for nuclear medicine procedures, and it is used in combination with LT4 in patients who do not respond to the standard treatment for hypothyroidism. This therapy is commonly done by using fixed doses, potentially resulting in supraphysiologic levels of triiodothyronine (T3). A good understanding of the LT3 pharmacokinetics (PK) is necessary to design combination treatment schemes that are able to maintain serum T3 levels within the reference range, but data on the PK of LT3 are conflicting. Here, we present a study designed to characterize the PK of LT3 in patients devoid of endogenous thyroid hormone production, and not receiving LT4 therapy. Methods: We performed an open-label, PK study in patients undergoing thyroid hormone withdrawal in preparation for nuclear medicine procedures for the evaluation and treatment of follicular-derived TC. LT3 was substituted for LT4 at a 1:3 mcg/mcg dosage ratio thrice daily for at least 30 days. PK of the last LT3 dose while at steady state and terminal elimination was assessed over 11 days. Thereafter, a PK study was performed following the nuclear medicine procedure in patients who volunteered for a second study. Results: Fourteen patients age 48.5 ± 16.0 years completed the last dose study and five completed the second PK study. PK analysis indicates a time to maximum serum concentration of 1.8 ± 0.32 hours and two distinct phases of linear elimination, with a fast distribution phase and slow elimination phases with half-lives of 2.3 ± 0.11 hours and 22.9 ± 7.7 hours, supporting a two-compartment model. PK modeling predicts that a twice-daily administration of low-dose LT3 (0.07 mcg/kg twice daily) in combination with LT4 can predictably increase the serum T3 concentration without significant peaks above the reference range. Conclusions: The PK of LT3 is well described by a two-compartment model that assumes elimination only from the sampling compartment, with a rapid distribution phase and a slow elimination phase. This information will contribute to design therapeutic strategies for LT3/LT4 combination therapies directed to maintain stable T3 serum levels.

Human obesity is an increasing burden worldwide. Lipids stored as metabolic fuel in white adipose tissue (WAT) can be rapidly mobilized through lipolysis. In rodent adipocytes, activation of the β3-adrenergic receptors (β3-ARs) leads to lipolysis. The physiological role of the β3-AR in human adipocytes remains controversial due to its reported low expression and previous access only to partial β3-AR agonists. Mirabegron, an FDA-approved drug for overreactive bladder, is a highly-selective human β3-AR agonist. In clinical trials, mirabegron treatment led to activation of human brown adipose tissue (BAT) thermogenesis and increased plasma NEFA’s. Therefore, we investigated if mirabegron directly mediates lipolysis in human white adipocytes. We found similar gene expression of the three β-ARs in human immortalized and primary white adipocytes, with β2-AR being the most expressed, followed by β1-AR and β3-AR. Next, to assess the functionally of the β3-AR in human adipocytes, we established dose-response curves and found that mirabegron increased the release of glycerol, a lipolytic product, in the picomolar range. To identify the molecular mechanism controlling mirabegron-induced lipolysis, we assessed the regulation of key downstream lipolytic proteins. Mirabegron mediated lipolysis by rapidly phosphorylating hormone sensitive lipase (HSL), as well as the mitogen activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK). Moreover, acute inhibition of either the PKA or MAPK/ERK pathways decreased lipolysis. In summary, we identify a major role played by the β3-AR in the regulation of human WAT lipolysis. Mirabegron acts as a potent pro-lipolytic effector, signaling through the canonical adrenergic receptor pathway as well as the MAPK/ERK pathway. Due to the comparatively minimal effects previously reported of mirabegron on the cardiovascular system, mirabegron may be the optimal choice for targeted lipid mobilization in human WAT. Disclosure C. Cero: None. J.W. Johnson: None. A. O’Mara: None. J.D. Linderman: None. A.M. Cypess: None. Funding National Institutes of Health

Background Liothyronine (LT3) is a substitute for levothyroxine (LT4) for thyroid cancer patients during the preparation for nuclear medicine procedures, and there is renewed interest in its use in combination with LT4 in patients who do not respond to the standard treatment. This therapy is commonly done on fixed doses potentially resulting in supraphysiologic levels of T3. A good understanding of the LT3 pharmacokinetics (PK) is necessary to design combination treatment schemes able to maintain serum T3 levels within normal range, but data on the PK of LT3 are conflicting. Here we present a study designed to characterize the PK of LT3 in patients devoid of endogenous thyroid hormone production, not receiving LT4. Methods We performed an open label PK study in patients undergoing thyroid hormone withdrawal in preparation for nuclear medicine procedures for the evaluation and treatment of follicular-derived thyroid cancer (Clinicaltrials.gov ID NCT01441154). LT3 was substituted for LT4 at 1:3 mcg/mcg dosage ratio thrice daily for at least thirty days. PK of single dose LT3 and terminal elimination were assessed over eleven days and first dose PK was offered following the nuclear medicine procedures. Results Fourteen patients (7 males, 7 females) age 48.5±16.0 years completed the study. The LT3 daily dose was 53.7±12.3 (0.74±0.1 mcg/Kg). PK studies performed with the morning dose (18.8±4.4 mcg), indicate a Cmax of 320±60 ng/dl, significantly above the upper limit (90-215). Tmax was 1.8±0.5 hours and two distinct phases of linear elimination with a fast distribution phase and slow elimination phases with half-lives of 1.2 hours and 29.9 hours were identified, supporting a two-compartment model. PK modeling for a 50 mcg LT3 total dose predicted that while the mean T3 levels would not significantly differ among single, twice, or thrice daily administration regimens, the variance would differ dramatically, 213.5±126 vs. 214.5±61 vs. 214.5±35 ng/dl, single, twice and thrice daily administration, respectively. We next performed a PK modeling for LT3/LT4 combination therapy, twice daily administration. The results indicate that low dose LT3 (up to 0.07 mcg/kg twice daily) in combination with LT4 can predictably increase the serum T3 concentration without significant peaks above the normal range. Conclusions The PK of LT3 is well described by a two-compartment model that assumes elimination only from the sampling compartment, with a rapid distribution phase and a slow elimination phase. This information will contribute to design therapeutic strategies for LT3/LT4 combination therapy directed to maintain stable T3 serum levels. Unless otherwise noted, all abstracts presented at ENDO are embargoed until the date and time of presentation. For oral presentations, the abstracts are embargoed until the session begins. Abstracts presented at a news conference are embargoed until the date and time of the news conference. The Endocrine Society reserves the right to lift the embargo on specific abstracts that are selected for promotion prior to or during ENDO.

Background: Brown Adipose Tissue (BAT) in adult humans has become increasingly recognized to contribute to energy consumption and modulate metabolism. To date, BAT stimulation has predominately relied on cold exposure. However, long-term cold exposure is impractical as a treatment option for obesity and diabetes. Thus, we investigated if a chronic pharmacotherapeutic approach could stimulate BAT and contribute to metabolic health. Methods: In an ongoing study, 6 healthy female volunteers received 4 weeks of daily mirabegron (Myrbetriq, Astellas Pharma) 100 mg, a β3 adrenergic receptor (AR) agonist. Acute responses were determined by comparing effects before and then five hours after dosing. Chronic changes were assessed after 4 weeks of treatment. The primary endpoint was the change in BAT metabolic activity measured by 18F-FDG PET/CT. Secondary endpoints included resting energy expenditure (REE), insulin and glucose sensitivity, liver steatosis, and cardiovascular stimulation. Results: Acute effects of mirabegron included consistent increases in rate pressure product (RPP), REE, and serum non-esterified fatty acids (NEFA) (p Conclusions: Acute treatment with mirabegron in women successfully activates BAT thermogenesis. Preliminary data show chronic treatment resulted in a blunting of several responses, consistent with physiological attenuation of β-AR signaling. The increases in sleeping EE and improved insulin sensitivity suggest chronic mirabegron treatment may help alleviate metabolic disease. Disclosure A. O'Mara: None. A. Cypess: None. C. Cero: None. J.W. Johnson: None. J.D. Linderman: None. B. Leitner: None. L. Fletcher: None. R. Brychta: None. D. Kapuria: None. S. McGehee: None. Y. Rotman: None.

Obesity results from the accumulation of excess white adipose tissue, and its increasing rates worldwide pose a significant risk to overall health due to its multiple comorbidities, like type 2 diabetes. A novel treatment strategy is to increase energy expenditure through activation of brown and beige adipocytes, which can use uncoupling protein 1 (UCP1) for thermogenesis. In addition to having different developmental lineages, brown and beige adipocytes differ functionally regarding the additional mechanisms by which they generate heat. Most human fat depots are not easily accessible, so little is known about their developmental and functional characteristics. In this study, we collected adipose tissue from 11 patients (age 16-84, 4 female/7 male) who had undergone autopsy. We sampled from 7 anatomically distinct fat depots: subcutaneous (sc), omental (om), retroperitoneal (rp), pericardial (pc), periadrenal (pa), paraspinal (ps), and supraclavicular (sv). We measured mRNA levels of lineage and functional genes associated with brown, beige, and white adipocytes. Energy storing white adipocytes were defined by high leptin expression, while energy expending brown and beige cells were defined by high UCP1; these were further distinguished as having a brown lineage using Zic1 and beige via Tbx1. Principal component analysis showed three different clusters of genes consistent with the brown, beige, and white lineages. Functionally, based on expression of leptin and UCP1 for white and brown fat, respectively, the sc and om were white (P In summary, human adipose tissue is not merely white or brown but instead is composed of many depots with distinct developmental lineages and functional capacities. These differences will be important when trying to use brown and beige adipocyte thermogenesis to treat obesity and diabetes. Disclosure J.W. Johnson: None. C. Cero: None. A. O'Mara: None. J.D. Linderman: None. A.S. Baskin: None. A. Cypess: None.

In rodent models, the β3-adrenergic receptor (AR) is responsible for thermogenesis in brown adipose tissue (BAT) and lipolysis in white adipose tissue (WAT). In contrast, the contribution of β3-AR in human adipocytes is controversial due to the low expression of the β3-AR in WAT and access only to partial β3-AR agonists. Mirabegron, an FDA-approved drug for overreactive bladder, is a selective human β3-AR agonist. In a recent clinical trial, mirabegron activated human BAT thermogenesis and WAT lipolysis. Therefore, we investigated the distribution and relative contribution of β1/2/3-AR in (i) whole white (hWAT) and brown (hBAT) adipose tissue from human autopsies and (ii) immortalized human white (hWA) and brown (hBA) adipocytes. Among the three β-ARs, β1-AR and β2-AR are similarly expressed in brown and white adipose tissue, whereas β3-AR has the highest expression in hBAT. In immortalized differentiated cells, hBA have a significantly higher expression of β3-AR compared to hWA. To assess the functionally of these receptors in adipocytes, dose-response curves of agonist-stimulated lipolysis were investigated in differentiated human and mouse adipocytes. We observed that stimulation of any one of the three β-ARs in human adipocytes induces a significant increase in glycerol release, a lipolytic byproduct. However, the effective dose of mirabegron to activate β3-AR-mediated lipolysis in white and brown adipocytes via mirabegron was significantly lower (hWA: EC50 2.55E-09; hBA: EC50 6.31E-09) compared to specific activators of β1-AR or β2-AR. In summary, we identify a major role played by the β3-AR in the regulation of human BAT thermogenesis and WAT lipolysis, both in vivo and in vitro. Due to the comparatively minimal effects of mirabegron treatment on the cardiovascular system, selective β3-AR agonists may be the optimal choice for targeted stimulation of human WAT and BAT. Disclosure C. Cero: None. A. O'Mara: None. J.W. Johnson: None. A.S. Baskin: None. J.D. Linderman: None. A. Cypess: None.

β3-adrenergic receptor (AR) agonists are approved to treat only overactive bladder. However, rodent studies suggest that these drugs could have other beneficial effects on human metabolism. We performed tissue receptor profiling and showed that the human β3-AR mRNA is also highly expressed in gallbladder and brown adipose tissue (BAT). We next studied the clinical implications of this distribution in 12 healthy men given one-time randomized doses of placebo, the approved dose of 50 mg, and 200 mg of the β3-AR agonist mirabegron. There was a more-than-dose-proportional increase in BAT metabolic activity as measured by [18F]-2-fluoro-D-2-deoxy-d-glucose positron emission tomography/computed tomography (medians 0.0 vs. 18.2 vs. 305.6 mL ⋅ mean standardized uptake value [SUVmean] ⋅ g/mL). Only the 200-mg dose elevated both nonesterified fatty acids (68%) and resting energy expenditure (5.8%). Previously undescribed increases in gallbladder size (35%) and reductions in conjugated bile acids were also discovered. Therefore, besides urinary bladder relaxation, the human β3-AR contributes to white adipose tissue lipolysis, BAT thermogenesis, gallbladder relaxation, and bile acid metabolism. This physiology should be considered in the development of more selective β3-AR agonists to treat obesity-related complications.

In rodents, brown adipose tissue (BAT) regulates cold- and diet-induced thermogenesis (CIT; DIT). Whether BAT recruitment is reversible and how it impacts on energy metabolism have not been investigated in humans. We examined the effects of temperature acclimation on BAT, energy balance, and substrate metabolism in a prospective crossover study of 4-month duration, consisting of four consecutive blocks of 1-month overnight temperature acclimation (24°C [month 1] → 19°C [month 2] → 24°C [month 3] → 27°C [month 4]) of five healthy men in a temperature-controlled research facility. Sequential monthly acclimation modulated BAT reversibly, boosting and suppressing its abundance and activity in mild cold and warm conditions (P < 0.05), respectively, independent of seasonal fluctuations (P < 0.01). BAT acclimation did not alter CIT but was accompanied by DIT (P < 0.05) and postprandial insulin sensitivity enhancement (P < 0.05), evident only after cold acclimation. Circulating and adipose tissue, but not skeletal muscle, expression levels of leptin and adiponectin displayed reciprocal changes concordant with cold-acclimated insulin sensitization. These results suggest regulatory links between BAT thermal plasticity and glucose metabolism in humans, opening avenues to harnessing BAT for metabolic benefits.

SummaryRediscovery of cold-activated brown adipose tissue (BAT) in humans has boosted research interest in identifying BAT activators for metabolic benefits. Of particular interest are cytokines capable of fat browning. Irisin, derived from FNDC5, is an exercise-induced myokine that drives brown-fat-like thermogenesis in murine white fat. Here we explored whether cold exposure is an afferent signal for irisin secretion in humans and compared it with FGF21, a brown adipokine in rodents. Cold exposure increased circulating irisin and FGF21. We found an induction of irisin secretion proportional to shivering intensity, in magnitude similar to exercise-stimulated secretion. FNDC5 and/or FGF21 treatment upregulated human adipocyte brown fat gene/protein expression and thermogenesis in a depot-specific manner. These results suggest exercise-induced irisin secretion could have evolved from shivering-related muscle contraction, serving to augment brown fat thermogenesis in concert with FGF21. Irisin-mediated muscle-adipose crosstalk may represent a thermogenic, cold-activated endocrine axis that is exploitable in obesity therapeutics development.

Thyroid hormones are important determinants of energy expenditure, and in rodents, adipose tissue affects thyroid hormone homeostasis via leptin signaling. The relationship between thyroid hormones and nutritional status in humans has been assessed primarily in drastic dietary or bariatric surgery interventions, while limited information is available on serial assessment of this axis during moderate, prolonged dietary restriction.To evaluate the effects of moderate dietary restriction on thyroid hormone homeostasis, 47 subjects with a body mass index (BMI) of 25-45 kg/m(2) were enrolled in a longitudinal intervention study; 30 nonoverweight volunteers were also enrolled as controls. Overweight and obese subjects underwent a 12-month individualized dietary intervention aimed at achieving a 5-10% weight loss.The intervention resulted in a 6.3±0.9 kg (6.5±1.0%) weight loss. At baseline, thyrotropin (TSH) and T3 concentrations correlated significantly with fat mass (R=0.257, p=0.024 and R=0.318, p=0.005, respectively). After weight loss, T3 decreased significantly (from 112.7±3.1 to 101.8±2.6 ng/dL, p0.001) in the absence of significant changes in TSH or free T4 (fT4). The decrease in serum T3 correlated with the decrease in weight (R=0.294, p0.001). The T3:fT4 ratio decreased significantly (p=0.02) in individuals who lost5% body weight.T3 concentration closely correlates with individual nutritional status, and moderate weight loss results in a decrease in T3 with minimal changes in other thyroid hormone homeostasis parameters. The data suggest that a decrease in peripheral conversion of the prohormone T4 into its hormonally active metabolite T3 is at least in part responsible for the observed changes in thyroid hormone homeostasis.

In animals, high fibroblast growth factor 21 (FGF21) states improve insulin resistance but induce bone loss. Whether FGF21 relates to bone mineral density (BMD) is unknown in humans. Contrary to prediction from animal findings, we found higher FGF21 levels associating with greater BMD in women, independent of age and body composition.Recent laboratory studies suggest that FGF21 is involved in reciprocal regulation of bone and energy homeostasis. Systemic administration of FGF21 protects animals from obesity and diabetes but causes severe bone loss, smothering the enthusiasm over FGF21 as a potential antiobesity therapeutic. To date, there is no information on whether FGF21 relates to BMD in humans. We thus studied the relationship between plasma FGF21 levels and BMD in healthy adults.Fasting plasma FGF21 levels were measured by enzyme-linked immunosorbent assay and body composition by dual-energy X-ray absorptiometry.Among 40 healthy volunteers (age 32 ± 10 year, 16 women), men had significantly higher lean body mass (p 0.01) and total BMD (p 0.05), and lower percent body fat than women (p 0.01). Median plasma FGF21 levels were not different between the sexes. While there was no association between FGF21 concentrations and body composition in men, FGF21 levels correlated positively with fat mass (p 0.01) in women. In men, no significant correlation between FGF21 with BMD was observed. However, in women, FGF21 correlated positively with total BMD (R (2) = 0.69, p = 0.003) and spine BMD (R (2) = 0.76, p = 0.001); the correlation remained significant after adjusting for age, ethnicity, and body composition.This study reveals for the first time a strong positive association between plasma FGF21 levels and BMD in healthy women, suggesting the association between bone loss and high FGF21 states in animals may not be directly translated to humans in physiologic states. We hypothesize that FGF21 may increase bone mass particularly in women through paracrine mechanisms in the bone-adipose interface.

The contribution of brown adipose tissue (BAT) to the energy balance in humans exposed to sustainable cold has not been completely established, partially because of measurement limitations of both BAT activity and energy expenditure (EE).The objective of the study was to characterize the role of BAT activation in cold-induced thermogenesis (CIT).This study was a single-blind, randomized crossover intervention.The study was conducted at the National Institutes of Health Clinical Center.Thirty-one healthy volunteers participated in the study.The intervention included mild cold exposure.CIT and BAT activation were the main outcomes in this study.Overnight EE measurement by whole-room indirect calorimeter at 24 °C or 19 °C was followed by 2-[18F]-fluoro-2-deoxy-D-glucose positron emission tomography (PET) scan. After 36 hours, volunteers crossed over to the alternate study temperature under identical conditions. BAT activity was measured in a 3-dimensional region of interest in the upper torso by comparing the uptake at the two temperatures.Twenty-four volunteers (14 males, 10 females) had a complete data set. When compared with 24 °C, exposure at 19 °C resulted in increased EE (5.3 ± 5.9%, P.001), indicating CIT response and mean BAT activity (10.5 ± 11.1%, P.001). Multiple regression analysis indicated that a difference in BAT activity (P.001), age (P = .01), and gender (P = .037) were independent contributors to individual variability of CIT.A small reduction in ambient temperature, within the range of climate-controlled buildings, is sufficient to increase human BAT activity, which correlates with individual CIT response. This study uncovers for the first time a spectrum of BAT activation among healthy adults during mild cold exposure not previously recognized by conventional PET and PET-computed tomography methods. The enhancement of cold-induced BAT stimulation may represent a novel environmental strategy in obesity treatment.

In animals, defective brown adipogenesis leads to bone loss. Whether brown adipose tissue (BAT) mass relates to bone mineral density (BMD) in humans is unclear. We determined the relationship between BAT mass and BMD by cold-stimulated positron-emission tomography (PET) and dual-energy X-ray absorptiometry (DXA) in healthy volunteers. Higher BAT mass was associated with higher BMD in healthy women, but not in men, independent of age and body composition. Contrary to the traditional belief that BAT is present only in infants, recent studies revealed significant depots of BAT present in adult humans. In animals, defective brown adipogenesis leads to bone loss. While white adipose tissue mass is a known determinant of BMD in humans, the relationship between BAT and BMD in humans is unclear. We thus examined the relationship between BAT and BMD in healthy adults. BAT volume (ml) and activity (standard uptake value) were determined by 18F-fluorodeoxyglucose PET after overnight mild cold exposure at 19 °C, and BMD was determined by DXA. Among 24 healthy adults (age 28 ± 1 years, F = 10), BAT volumes were 82.4 ± 99.5 ml in women and 49.7 ± 54.5 ml in men. Women manifested significantly higher BAT activity, by 9.4 ± 8.1 % (p = 0.03), than men. BAT volume correlated positively with total and spine BMD (r 2 = 0.40 and 0.49, respectively, p

The substitution of liothyronine (L-T3) for levothyroxine (L-T4) is commonly employed during thyroid hormone (TH) withdrawal in preparation for diagnostic and therapeutic interventions on thyroid cancer patients. Presently, only limited data are available on the L-T3 for L-T4 therapeutic substitution. Objective To characterize the pharmcodynamic equivalence of L-T3 and L-T4.Randomized, double-blind, cross-over intervention study.NIH clinical center.Ten thyroidectomized patients.Study participants were treated with L-T3 or L-T4 with a target TSHor= 0.5or= 1.5 mU/l for at least 30 days before undergoing inpatient testing. Following testing, subjects crossed-over according to the same scheme.Area under the serum concentration-time curve of TSH from 0 to 60 min (AUC(0-60)) and peak TSH serum concentration (C(max)) following thyrotropin-releasing hormone (TRH) stimulation test, total L-T4 and L-T3 dose (mcg/kg), and L-T4/L-T3 ratio.No difference was observed for time 0 TSH values between L-T3 and L-T4 replacement phases (1.48 +/- 0.77 vs. 1.21 +/- 0.62 mU/l, P = 0.293) at average daily doses of 40.3 +/- 11.3 mcg L-T3 and 115.2 +/- 38.5 mcg L-T4, L-T3: L-T4 ratio 0.36 +/- 0.06. TRH stimulation test resulted in similar L-T3 vs. L-T4 TSH responses with AUC(0-60) of 326.1 (95% CI 232.6-457.1) and 247.1 (95% CI 153.8-397.1) mU* min/l (P = 0.285); and C(max) of 6.83 (95% CI 4.88-9.55) and 5.23 (95% CI 3.31-8.3) mU/l (P = 0.383).This is the first study addressing the equivalency between L-T3 and L-T4 therapy measured by baseline and TRH-stimulated TSH. The therapeutic substitution of L-T3 for L-T4 was achieved at approximately 1:3 ratio.

Weight loss intervention is the principal non-pharmacological method for prevention and treatment of type 2 diabetes. However, little is known whether it influences insulin sensitivity directly or via its anti-inflammatory effect. The aim of this study was to assess the independent role of changes in inflammation status and weight loss on insulin sensitivity in this population.Overweight and obese nondiabetic participants without co-morbidities underwent a one-year weight loss intervention focused on caloric restriction and behavioral support. Markers of inflammation, body composition, anthropometric para-meters, and insulin sensitivity were recorded at baseline, 6, and 12 months. Insulin sensitivity was assessed with frequently sampled intravenous glucose tolerance test and Minimal Model. Twenty-eight participants (F: 15, M: 13, age 39±5 years, BMI 33.2±4.6 kg/m(2)) completed the study, achieving 9.4±6.9% weight loss, which was predominantly fat mass (7.7±5.6 kg, p0.0001). Dietary intervention resulted in significant decrease in leptin, leptin-to-adiponectin ratio, hs-CRP, and IL-6 (all p0.02), and improvement in HOMA-IR and Insulin Sensitivity Index (SI) (both p0.001). In response to weight loss IL-1β, IL-2, leptin, and resistin were significantly associated with insulin, sensitivity, whereas sICAM-1 had only marginal additive effect. Moderate weight loss in otherwise healthy overweight and obese individuals resulted in an improvement in insulin sensitivity and in the overall inflammation state; the latter played only a minimal independent role in modulating insulin sensitivity.

A recent trial showed that 1:3 μg:μg liothyronine (L-T3) substitution for levothyroxine (L-T4) achieving near-identical TSH levels resulted in a significant decrease in weight and cholesterol levels with no appreciable changes in cardiovascular parameters, suggesting a differential peripheral response to the therapy.We characterized the pituitary-thyroid axis in hypothyroid patients receiving equivalent doses of L-T3 or L-T4 by escalating-dose TRH stimulation test.A secondary analysis of a L-T3 vs L-T4 therapy trial was performed.The study was conducted at the National Institutes of Health.Thirteen patients were studied.Escalating-dose (5, 15, and 200 μg) TRH stimulation test on both treatment arms.Study outcomes were peak serum TSH concentration (Cmax), time to peak TSH concentration (Tmax), area under the curve from 0 to 60 minutes (AUC₀₋₆₀) after TRH injection.Thirteen patients aged 51.2 ± 8.29 years completed escalating-dose TRH stimulation test. No significant difference between L-T3 and L-T4 treatments was observed in TSH Cmax or area under the curve. L-T4 resulted in a small but significantly shorter Tmax compared to L-T3 (3.5 ± 0.73 min on 200 μg TRH dose, P.03). In addition, 5 μg TRH dose compared to 200 μg resulted in a shorter Tmax on both treatment arms (6.9 ± 0.59 min L-T3, 4 ± 0.3 min L-T4; P = .0002).The assessment of the dynamic pituitary response to escalating doses of TRH confirms that substitution of L-T3 for L-T4 on a 1:3 ratio achieves a near-identical degree of pituitary euthyroidism. Furthermore, the data suggest that lower doses of TRH might provide clinically relevant information of thyrotroph function, particularly when investigating partial pituitary insufficiency states.

Cold exposure stimulates fibroblast growth factor 21 (FGF21) secretion in animals, enhancing the cold-induced thermogenesis (CIT) response through browning of white adipose tissue. In humans, the effects of cold exposure on circulating FGF21 levels are unknown.Our objective was to evaluate the effects of mild cold exposure on circulating FGF21 and its relationship with CIT and lipolysis in humans.We conducted a randomized, single-blind, crossover intervention study at the National Institutes of Health Clinical Center.Participants were healthy adults.Subjects were exposed to a 12-h exposure to 24 or 19 C in a whole-room indirect calorimeter.Energy expenditure, plasma FGF 21, nonesterified fatty acid, and adipose tissue microdialysis glycerol concentrations were evaluated.At 24 C, plasma FGF21 exhibited a diurnal rhythm, peaking at 0800 h [110 (59-178) pg/ml], and progressively dropped to a nadir at 1700 h [41 (21-71) pg/ml, P0.0001] before rising at 1900 h [60 (11-81) pg/ml, P0.0001]. Exposure at 19 C lessened the diurnal reduction of FGF21 observed at 24 C from 0800-1700 h and augmented overall FGF21 levels by 37 ± 45% (P = 0.01). The change in area under the curve plasma FGF21 between 19 and 24 C correlated positively with the change in area under the curve adipose microdialysate glycerol (R(2) = 0.35, P = 0.04) but not with nonesterified fatty acid. Cold-induced increase in FGF21 predicted greater rise in energy expenditure during cold exposure (β = 0.66, P = 0.027), independent of age, gender, fat mass, and lean mass.Mild cold exposure increased circulating FGF21 levels, predicting greater lipolysis and CIT. A small reduction in environmental temperature is sufficient to modulate FGF21 diurnal rhythm in humans, which may mediate cold-induced metabolic changes similar to those in animals.

Objective: Type 2 deiodinase gene (DIO2) polymorphisms have been associated with changes in pituitary–thyroid axis homeostasis. The K258A/G (SNP rs12885300) polymorphism has been associated with increased enzymatic activity, but data are conflicting. To characterize the effects of K258A/G polymorphism on intrathyroidal thyroxine (T4) to triiodothyronine (T3) conversion and thyroid hormone (TH) secretion pattern, we studied the effects of acute, TRH-mediated, TSH stimulation of the thyroid gland. Design: Retrospective analysis. Methods: The TH secretion in response to 500 mg i.v. TRH injection was studied in 45 healthy volunteers. Results: Twenty-six subjects (16 females and ten males, 32.8G10.4 years) were homozygous for the ancestral (K258A/A) allele and 19 (11 females and eight males, 31.1G10.9 years) were carriers of the (K258G/x) variant. While no differences in the peak TSH and T3 levels were observed, carriers of the K258G/x allele showed a blunted rise in free T4 (FT4; P!0.01). The K258G/x92Thr/Thr haplotype, compared with the other groups, had lower TSH values at 60 min (P!0.03). No differences were observed between genotypes in baseline TH levels. Conclusions: The K258G/x DIO2 polymorphism variant is associated with a decreased rate of acute TSH-stimulated FT4 secretion with a normal T3 release from the thyroid gland consistent with a shift in the reaction equilibrium toward the product. These data indicate that the K258G DIO2 polymorphism causes changes in the pattern of hormone secretion. These findings are a proof of concept that common polymorphisms in DIO2 can subtly affect the circulating levels of TH and might modulate the TH homeostasis.

Levothyroxine (L-T(4)) therapy is based on the assumption that the conversion of T(4) into T(3) provides adequate amounts of active hormone at target tissues. However, in rodents, L-T(4) alone does not restore a euthyroid state in all tissues. Previous combination L-T(4)/liothyronine (L-T(3)) therapy trials focused on quality-of-life endpoints, and limited information is available on the effects on other measures of thyroid hormone action.Our objective was to evaluate the efficacy of thyroid hormone replacement with L-T(4) or L-T(3) at doses producing equivalent normalization of TSH.Fourteen hypothyroid patients participated in this randomized, double-blind, crossover intervention at the National Institutes of Health Clinical Center.L-T(3) or L-T(4) were administered thrice daily to achieve a target TSH from 0.5-1.5 mU/liter. Volunteers were studied as inpatients after 6 wk on a stable dose and at the target TSH.Serum thyroid hormones, lipid parameters, and indices of glucose metabolism were evaluated.No difference was observed in TSH between L-T(3) and L-T(4) treatments. L-T(3) resulted in significant weight loss [L-T(4), 70.6 ± 12.5, vs. L-T(3), 68.5 ± 11.9 kg (P = 0.009)] and in a 10.9 ± 10.0% decrease in total cholesterol (P = 0.002), 13.3 ± 12.1% decrease in low-density lipoprotein-cholesterol (P = 0.002), and an 18.3 ± 28.6% decrease in apolipoprotein B (P = 0.018). No significant differences were observed in high-density lipoprotein-cholesterol, heart rate, blood pressure, exercise tolerance, or insulin sensitivity.The substitution of L-T(3) for L-T(4) at equivalent doses (relative to the pituitary) reduced body weight and resulted in greater thyroid hormone action on the lipid metabolism, without detected differences in cardiovascular function or insulin sensitivity.

The common Thr92Ala D2 polymorphism has been associated with changes in pituitary-thyroid axis homeostasis, but published results are conflicting. To investigate the effects of the Thr92Ala polymorphism on intrathyroidal thyroxine (T4) to triiodothyronine (T3) conversion, we designed prospective pharmacogenomic intervention aimed to detect differences in T3 levels after thyrotropin (TSH)-releasing hormone (TRH)-mediated TSH stimulation of the thyroid gland.Eighty-three healthy volunteers were screened and genotyped for the Thr92Ala polymorphism. Fifteen volunteers of each genotype (Thr/Thr, Thr/Ala, and Ala/Ala) underwent a 500 mcg intravenous TRH stimulation test with serial measurements of serum total T3 (TT3), free T4, and TSH over 180 minutes.No differences in baseline thyroid hormone levels were seen among the study groups. Compared to the Thr/Thr group, the Ala/Ala group showed a significantly lower TRH-stimulated increase in serum TT3 at 60 minutes (12.07 ± 2.67 vs. 21.07 ± 2.86 ng/dL, p = 0.029). Thr/Ala subjects showed an intermediate response. Compared to Thr/Thr subjects, the Ala/Ala group showed a blunted rate of rise in serum TT3 as measured by mean time to 50% maximum delta serum TT3 (88.42 ± 6.84 vs. 69.56 ± 6.06 minutes, p = 0.028). Subjects attained similar maximal (180 minutes) TRH-stimulated TT3 levels. TRH-stimulated TSH and free T4 levels were not significantly different among the three genotype groups.The commonly occurring Thr92Ala D2 variant is associated with a decreased rate of acute TSH-stimulated T3 release from the thyroid consistent with a decrease in intrathyroidal deiodination. These data provide a proof of concept that the Thr92Ala polymorphism is associated with subtle changes in thyroid hormone homeostasis.

ObjectiveResting energy expenditure (EE) is a major contributor to the total EE and thus plays an important role in body weight regulation. Adaptive thermogenesis is a major component of EE in rodents, but little is known on the effects of exposure of humans to mild and sustainable reduction in environmental temperature.DesignTo characterize the dynamic changes in continuously measured resting EE, substrate utilization, and hormonal axes simultaneously in response to mild reduction in environmental temperature, we performed a cross-over intervention.MethodsTwenty-five volunteers underwent two 12-h recordings of EE in whole room indirect calorimeters at 24 and 19 °C with simultaneous measurement of spontaneous movements and hormonal axes.ResultsExposure to 19 °C resulted in an increase in plasma and urine norepinephrine levels (PPPPPPConclusionsOur observations indicate that exposure to 19 °C, a mild and tolerable cold temperature, results in a predictable increase in EE driven by a sustained rise in catecholamine and the activation of counter-regulatory mechanisms.