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Background/Aims: We aimed to evaluate the association between change in thyroid function tests within the euthyroid range and body mass index (BMI) in persons with normal weight at baseline. Methods: This study investigated 1,100 normal-weight euthyroid persons in a population-based cohort study, Tehran Thyroid Study. BMI was calculated and serum concentrations of thyrotropin (TSH) and free T4 (FT4) were assayed at baseline and after 10 years of follow-up. We evaluated the relationship between thyroid and obesity based on 2 definitions for outcome: (1) a binary outcome as BMI <25 or ≥ 25 kg/m2, and (2) a multinomial outcome as normal BMI, overweight, and obese. Results: A total of 569 women and 531 men, aged 36.3 ± 13.5 years, were included. Modified Poisson regression analysis for binary outcome, after adjustment for age, sex, smoking, and antithyroid peroxidase antibody status, revealed a negative association between delta serum FT4 and follow-up BMI (relative risk 0.55 [95% CI 0.37–0.80]) without any significant association between change in serum TSH and follow-up BMI. However, in multinomial logistic regression analysis, we found no relationship between delta serum FT4 or TSH and follow-up BMI categories, for either overweight or obese vs. normal-weight participants. Conclusions: In normal-weight euthyroid individuals, changes in serum concentrations of FT4, but not TSH, may contribute to change in body weight. [ABSTRACT FROM AUTHOR]

Background: Statins, such as simvastatin, are the drugs of choice for the treatment of hypercholesterolemia. On the other hand hypercholesterolmia can occur in hypothyroid patients, who receive levothyroxine. There are few clinical case reports in regards to drug interaction between levothyroxine and lovastatin or simvastatin, indicating decreased levothyroxine effects. This study aimed at determining possible interaction between simvastatin and levothyroxine in hypothyroid patients by assessing serum levels of thyroid stimulating hormone (TSH) and free thyroxine (FT4), the two important laboratory indices for levothyroxine therapy. Methods: In a cross sectional study, 41 eligible hypothyroid patients receiving levothyroxine (50-150 µg/d) were selected. Blood samples were taken before and after three months of simultaneous treatment with simvastatin (20 mg/d) and levothyroxine to determine the serum levels of TSH and FT4. Results: There was no significant difference between the serum levels of TSH (P=0.77) or FT4 (P=0.76) before and after three months of simultaneous treatment. Also, there was no aggravation or initiation of any sign or symptom of hypothyroidism in the patients during the study period. Conclusion: Considering that FT4 and TSH are the most reliable indicators for the levothyroxine treatment, the findings of the present study suggest that there may not be any significant interaction between simvastatin and levothyroxine. [ABSTRACT FROM AUTHOR]

Aim: The association between insulin resistance and thyroid function in euthyroid subjects has not yet been clarified. This study aimed to investigate the association between thyroid function within the normal reference range and insulin resistance in participants of the Tehran Thyroid Study (TTS)., Methods: This cross-sectional study was conducted within the framework of the TTS. Of 5786 subjects aged ≥ 20 years, 2758 euthyroid subjects free of thyroid disorders, diabetes, chronic kidney disease and cardiovascular disease, and not taking steroids and lipid-lowering agents, were included. Serum concentrations of free thyroxine (FT4) and TSH were measured. The homoeostasis model assessment index for insulin resistance (HOMA-IR) was used to evaluate IR., Results: On linear regression analysis, a negative association was found between serum FT4 levels and HOMA-IR in the model with age, smoking and physical activity (B = -0.09, P < 0.001) and in the WC-adjusted model with age, smoking and physical activity for men (B = -0.06, P < 0.01). In addition, there was a positive association between serum TSH levels and HOMA-IR in both models [with age, smoking and physical activity (B = 0.07, P = 0.006), and age, smoking, physical activity and adjusted for WC (B = 0.05, P = 0.01)] that was not more significant on logistic regression analysis. In women, neither serum FT4 nor TSH levels were associated with HOMA-IR; the prevalence of IR decreased from 27.2 to 19.1 with increasing tertiles of FT4 only in men (P = 0.01). No significant differences were observed in HOMA-IR and its components between thyroid peroxidase antibody (TPOAb)-negative and -positive groups. Also, it was found that metabolically healthy but obese (MHO) subjects had higher levels of TSH than individuals who were MONW (metabolically obese but normal weight; P < 0.01)., Conclusion: Low FT4 was independently associated with IR in healthy euthyroid Iranian men., (Copyright © 2015 Elsevier Masson SAS. All rights reserved.)