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Background: L-triiodothyronine (LT3) has been increasingly used in combination with levothyroxine in the treatment of hypothyroidism. A metal coordinated form of LT3, known as poly-zinc-liothyronine (PZL), avoided in rats the typical triiodothyronine (T3) peak seen after oral administration of LT3. Objectives: To evaluate in healthy volunteers (i) the pharmacokinetics (PK) of PZL-derived T3 after a single dose, (ii) the pharmacodynamics of PZL-derived T3, (iii) incidence of adverse events, and (iv) exploratory analysis of the sleep patterns after LT3, PZL, or placebo (PB) administration. Methods: Twelve healthy volunteers 18–50 years of age were recruited for a Phase 1, double-blind, randomized, single-dose PB-controlled, crossover study to compare PZL against LT3 or PB. Subjects were admitted three separate times to receive a randomly assigned capsule containing PB, 50 μg LT3, or 50 μg PZL, and were observed for 48 hours. A 2-week washout period separated each admission. Results: LT3-derived serum T3 levels exhibited the expected profile, with a T(max) at 2 hours and return to basal levels by 24–36 hours. PZL-derived serum T3 levels exhibited ∼30% lower C(max) that was 1 hour delayed and extended into a plateau that lasted up to 6 hours. This was followed by a lower but much longer plateau; by 24 hours serum T3 levels still exceeded ½ of C(max). Thyrotropin levels were similarly reduced in both groups. Conclusion: PZL possesses the necessary properties to achieve a much improved T3 PK. PZL is on track to provide hypothyroid patients with stable levels of serum T3.

Background: Illness severity in patients infected with COVID-19 is variable. Methods: Here, we conducted an observational, longitudinal, and prospective cohort study to investigate serum thyroid hormone (TH) levels in adult COVID-19 patients, admitted between June and August 2020, and to determine whether they reflect the severity or mortality associated with the disease. Results: Two hundred forty-five patients [median age: 62 (49-75) years] were stratified into non-critical (181) and critically ill (64) groups. Fifty-eight patients (23.6%) were admitted to the intensive care unit, and 41 (16.7%) died. Sixteen (6.5%) exhibited isolated low levels of free triiodothyronine (fT3). fT3 levels were lower in critically ill compared with non-critical patients [fT3: 2.82 (2.46-3.29) pg/mL vs. 3.09 (2.67-3.63) pg/mL, p = 0.007]. Serum reverse triiodothyronine (rT3) was mostly elevated but less so in critically ill compared with non-critical patients [rT3: 0.36 (0.28-0.56) ng/mL vs. 0.51 (0.31-0.67) ng/mL, p = 0.001]. The univariate logistic regression revealed correlation between in-hospital mortality and serum fT3 levels (odds ratio [OR]: 0.47; 95% confidence interval [CI 0.29-0.74]; p = 0.0019), rT3 levels (OR: 0.09; [CI 0.01-0.49]; p = 0.006) and the product fT3 × rT3 (OR: 0.47; [CI 0.28-0.74]; p = 0.0026). Serum thyrotropin, free thyroxine, and fT3/rT3 values were not significantly associated with mortality and severity of the disease. A serum cutoff level of fT3 (≤2.6 pg/mL) and rT3 (≤0.38 ng/mL) was associated with 3.46 and 5.94 OR of mortality, respectively. We found three COVID-19 mortality predictors using the area under the receiver operating characteristic (ROC) curve (AUC score): serum fT3 (AUC = 0.66), rT3 (AUC = 0.64), and the product of serum fT3 × rT3 (AUC = 0.70). Non-thyroidal illness syndrome (fT3 < 2.0 pg/mL) was associated with a 7.05 OR of mortality ([CI 1.78-28.3], p = 0.005) and the product rT3 × fT3 ≤ 1.29 with an 8.08 OR of mortality ([CI 3.14-24.2], p < 0.0001). Conclusions: This prospective study reports data on the largest number of hospitalized moderate-to-severe COVID-19 patients and correlates serum TH levels with illness severity, mortality, and other biomarkers to critical illness. The data revealed the importance of early assessment of thyroid function in hospitalized patients with COVID-19, given the good prognostic value of serum fT3, rT3, and fT3 × rT3 product. Further studies are necessary to confirm these observations.

Background: Iodothyronine deiodinase-1 (D1) selenoenzyme regulates the systemic supply of active thyroid hormone (TH). Transient decrease in D1 enzymatic activity is clinically relevant and adaptive in nonthyroidal illness such as fasting or acute illness. However, DIO1 gene defects have not been reported in humans. Methods: Genetic analysis was performed using whole-exome sequencing in members of two unrelated families presenting with abnormal serum thyroid function tests. Plasmid constructs containing the two pathogenic DIO1 variants were used for in vitro studies assessing the kinetics of their enzymatic activity. Thyroid function tests were measured in Dio1 heterozygous-null mice. Results: We report the novel identification and characterization of two missense DIO1 pathogenic variants (resulting in p.Asn94Lys and p.Met201Ile) in two unrelated families presenting with abnormal TH metabolism with elevated serum reverse triiodothyronine (rT3) levels and rT3/T3 ratios. These characteristic in vivo parameters are also present in Dio1 heterozygous-null mice. Kinetic studies of the resulting mutant D1 proteins demonstrate two- to threefold higher Km indicating lower substrate affinity and slower enzyme velocity. Conclusions: We report the identification and characterization of two missense DIO1 pathogenic variants identified in families with abnormal TH metabolism. This is the first demonstration of inherited D1 deficiency in humans.

A number of mechanisms modify thyroid hormone (TH) signaling at the cellular level. To restore TH signaling in patients with hypothyroidism or in patients with the syndrome of TH resistance, it is necessary to quantify the action of THs in a tissue-specific manner. The development of biomarkers that are tissue-specific and respond to TH is a significant first step toward understanding and possibly modifying TH signaling in health and disease states.

Background: Liothyronine (LT3) has limited short-term clinical applications, all of which aim at suppressing thyrotropin (TSH) secretion. A more controversial application is chronic administration along with levothyroxine in the treatment of hypothyroidism. Long-term treatment with LT3 is complicated by its unique pharmacokinetics that result in a substantial triiodothyronine (T3) peak in the blood three to four hours after oral dosing. This is a significant problem, given that T3 levels in the blood are normally stable, varying by

Myogenesis is positively regulated by thyroid hormone (triiodothyronine [T3]), which is amplified by the type 2 deiodinase (D2) activation of thyroxine to T3. Global inactivation of the Dio2 gene impairs skeletal muscle (SKM) differentiation and regeneration in response to muscle injury. Given that newborn and adult mice with late developmental SKM Dio2 disruption do not develop a significant phenotype, it was hypothesized that D2 plays an early role in this process.This was tested in mice with SKM disruption of Dio2 driven by two early developmental promoters: MYF5 and MYOD.MYF5 myoblasts in culture differentiate normally into myotubes, despite loss of almost all D2 activity. Dio2 mRNA levels in developing SKM obtained from MYF5-D2KO embryos (E18.5) were about 54% of control littermates, but the expression of the T3-responsive genes Myh1 and 7 and Atp2a1 and 2 were not affected. In MYF5-D2KO and MYOD-D2KO neonatal hind-limb muscle, the expression of Myh1 and 7 and Atp2a2 remained unaffected, despite 60-70% loss in D2 activity and/or mRNA. Only in MYOD-D2KO neonatal muscle was there a 40% reduction in Atp2a1 mRNA. Postnatal growth of both mouse models and SKM function as assessed by exercise capacity and measurement of muscle strength were normal. Furthermore, an analysis of the adult soleus revealed no changes in the expression of T3-responsive genes, except for an about 18% increase in MYOD-D2KO SOL Myh7 mRNA.Two mouse models of early developmental disruption of Dio2 in myocyte precursor exhibit no significant SKM phenotype.

Background: A number of recent advances in our understanding of thyroid physiology may shed light on why some patients feel unwell while taking levothyroxine monotherapy. The purpose of this task force was to review the goals of levothyroxine therapy, the optimal prescription of conventional levothyroxine therapy, the sources of dissatisfaction with levothyroxine therapy, the evidence on treatment alternatives, and the relevant knowledge gaps. We wished to determine whether there are sufficient new data generated by well-designed studies to provide reason to pursue such therapies and change the current standard of care. This document is intended to inform clinical decision-making on thyroid hormone replacement therapy; it is not a replacement for individualized clinical judgment. Methods: Task force members identified 24 questions relevant to the treatment of hypothyroidism. The clinical literature relating to each question was then reviewed. Clinical reviews were supplemented, when relevant, with related mechanistic and bench research literature reviews, performed by our team of translational scientists. Ethics reviews were provided, when relevant, by a bioethicist. The responses to questions were formatted, when possible, in the form of a formal clinical recommendation statement. When responses were not suitable for a formal clinical recommendation, a summary response statement without a formal clinical recommendation was developed. For clinical recommendations, the supporting evidence was appraised, and the strength of each clinical recommendation was assessed, using the American College of Physicians system. The final document was organized so that each topic is introduced with a question, followed by a formal clinical recommendation. Stakeholder input was received at a national meeting, with some subsequent refinement of the clinical questions addressed in the document. Consensus was achieved for all recommendations by the task force. Results: We reviewed the following therapeutic categories: (i) levothyroxine therapy, (ii) non-levothyroxine-based thyroid hormone therapies, and (iii) use of thyroid hormone analogs. The second category included thyroid extracts, synthetic combination therapy, triiodothyronine therapy, and compounded thyroid hormones. Conclusions: We concluded that levothyroxine should remain the standard of care for treating hypothyroidism. We found no consistently strong evidence for the superiority of alternative preparations (e.g., levothyroxine-liothyronine combination therapy, or thyroid extract therapy, or others) over monotherapy with levothyroxine, in improving health outcomes. Some examples of future research needs include the development of superior biomarkers of euthyroidism to supplement thyrotropin measurements, mechanistic research on serum triiodothyronine levels (including effects of age and disease status, relationship with tissue concentrations, as well as potential therapeutic targeting), and long-term outcome clinical trials testing combination therapy or thyroid extracts (including subgroup effects). Additional research is also needed to develop thyroid hormone analogs with a favorable benefit to risk profile.

Background: An in-depth understanding of the fundamental principles that regulate thyroid hormone homeostasis is critical for the development of new diagnostic and treatment approaches for patients with thyroid disease. Summary: Important clinical practices in use today for the treatment of patients with hypothyroidism, hyperthyroidism, or thyroid cancer are the result of laboratory discoveries made by scientists investigating the most basic aspects of thyroid structure and molecular biology. In this document, a panel of experts commissioned by the American Thyroid Association makes a series of recommendations related to the study of thyroid hormone economy and action. These recommendations are intended to promote standardization of study design, which should in turn increase the comparability and reproducibility of experimental findings. Conclusions: It is expected that adherence to these recommendations by investigators in the field will facilitate progress towards a better understanding of the thyroid gland and thyroid hormone dependent processes.