LACK OF EFFECTS OF CIRCULATING THYROID-HORMONE LEVELS ON SERUM LEPTINCONCENTRATIONS

Leptin is the protein product of the ob gene, secreted by adipocytes. It has been suggested that it may play an important role in regulating appetite and energy expenditure. The aim of this study was to evaluat e a possible interaction of thyroid hormones with the leptin system. W e studied 114 adult patients (65 females and 49 males): 36 were affect ed with primary hypothyroidism (PH), 38 with central hypothyroidism (C H) and 40 with thyrotoxicosis (TT). Patients with CH were studied both before and after 6 months of L-thyroxine replacement therapy Body mas s index (BMI; kg/m(2)), thyroid function and fasting serum leptin were assessed in all patients. Since BMI has been proved to be the major i nfluencing variable of circulating leptin levels, data were expressed as standard deviation score (SDS) calculated from 393 male and 561 fem ale controls matched for age and BMI. No difference in SDS was recorde d between males and females whatever the levels of circulating thyroid hormones. In males, no significant difference was recorded among the SDSs of PH (-0.36+/-1.2), TT (-0.35+/-1.2) and CH (0.01+/-1.4) patient s. Females with PH had an SDS significantly lower than TT females (-0. 77+/-1.0 vs -0.06+/-1.2; P < 0.02), while no significant differences b etween CH (-0.34+/-0.7) and TT females or between CH and PH females we re observed. SDS in CK patients after 6 months of L-thyroxine therapy significantly varied only in females (0.25+/-1.4). In conclusion, circ ulating thyroid hormones do not appear to play any relevant role in le ptin synthesis and secretion. However, as females with either overt hy po-or hyper-thyroidism or central hypothyroidism after L-thyroxine the rapy show differences in their SDSs, a subtle interaction between sex steroids and thyroid status in modulating leptin secretion, at least i n women, may occur.