Interactions of leptin and thyrotropin 24-hour secretory profiles in shortnormal children

Thyroid hormones and leptin have effects on similar aspects of body homeost asis, such as energy expenditure, thermogenesis, and metabolic efficiency. Thus, the cross-talk between the thyrostat and the lipostat might play a cr ucial role in the maintenance of body homeostasis. To investigate the relat ionship between the hypothalamic-pituitary-thyroid (HPT) axis and leptin un der physiological conditions, we evaluated the pulsatility and circadian rh ythmicity and time-cross-correlated the 24-h secretory patterns of leptin a nd TSH in 12 short normal prepubertal children (6 girls and 6 boys). In bot h male and female subjects, leptin was secreted in a pulsatile and circadia n fashion, with a nocturnal leptin surge that was more pronounced in males than in females. Mean 24-h leptin levels and total area under the curve wer e significantly higher in girls than in boys. This was mainly due to the ni ghttime mean leptin levels and total area under the curve, which were highe r than those in boys. The cross-correlated 24-h leptin and TSH levels revea led significant positive and negative correlations. The positive one, of le ptin over TSH, suggests a positive feedback regulation by leptin on the HPT axis, which might play an important role in triggering the neuroendocrine response to starvation, including decreased thyroid hormone levels. The neg ative correlation, of TSH over leptin, could explain the compensatory chang es in adipocyte metabolism, and indirectly in circulating leptin levels, in response to alterations in thyroid status. In conclusion, we suggest that under baseline physiological conditions, the HPT axis has a prevailing inhi bitory effect on leptin secretion, whereas leptin has a prevailing positive effect on the HPT axis. The sexual dimorphism in leptin levels does not se em to influence in a major way the interactions between the HPT axis and le ptin.