Serum leptin concentrations of patients with sequential thyroid function changes

BACKGROUND: Leptin, a recently discovered protein produced in adipocytes, regulates body weight by suppressing food intake and/or increasing energy expenditure. Thyroid hormones, which increase the basal metabolic rate and thermogenesis, have been reported to be one of leptin's regulating factors because alternations in thyroid status might lead to compensatory changes in circulating leptin. OBJECTIVE: The aim of this study was to assess the influence of sequential changes in thyroid function on serum leptin levels. PATIENTS AND METHODS: The thyroid function status of 65 patients (55 women and 10 men, aged 40.6 ± 15.2 years, mean ± SD) with differentiated thyroid cancer who had received near-total thyroidectomy was studied before I-131 ablation therapy and after 2-4 and 6 months of levo-thyroxine suppressive therapy. Thirty-three (26 women, seven men; aged 41.0 ± 10.4 years, mean ± SD) of them were found to have become hypothyroid, then euthyroid and subsequently subclinically hyperthyroid. Their body mass index (BMI), body fat (%BF) by electrical bioimpedance, thyroid function and fasting serum leptin in these states were assessed and compared to those of 38 controls (30 women, eight men, aged 40.2 ± 11.3 years, mean ± SD). The controls had no past history or family history of thyroid diseases, and had the same range of BMI, between 20 and 30 kg/m², as the patients. RESULTS: No difference in serum leptin levels was found between patients and controls with comparable age, sex, and BMI distribution in the euthyroid state. Using a repeated measures ANOVA, tests of TSH, free T4 (FT4), BMI, %BF and leptin were performed on the 33 patients with sex as a grouping factor and thyroid state as a time factor. The sex difference for %BF and leptin proved to be statistically significant (P < 0.0001, and P = 0.0003, respectively). Serum leptin levels increased significantly from the hypothyroid to the subclinical hyperthyroid state (P < 0.0001) with a more pronounced increase found in females than in males (P = 0.03). Change of BMI during sequential thyroid function alterations was significant (P = 0.04) while change in %BF was not significant (P = 0.09). Pearson correlation analysis showed that serum leptin levels significantly correlated with BMI, %BF, FT4, and TSH (all P < 0.05). Using the generalized estimating equations, multivariate regression analysis revealed that FT4 was a statistically independent predictor for serum leptin (P < 0.0001). While other parameters were held constant, the mean serum leptin level increased by 1.47 units when serum FT4 was increased by one unit. CONCLUSION: In conclusion, our data indicate that circulating thyroid hormone plays a relevant role in regulating leptin metabolism independent of BMI and body fat.