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This study was performed on 36 obese subjects aged 8.5-17.4 yr, 14 boys and 22 girls (prepubertal: 5 boys and 5 girls [stage I, according to Tanner]; BMI: 35.5 +/- 1.4 [mean +/- SEM] and 35 +/- 1.3 respectively; pubertal: 9 boys and 17 girls [stage IV-V]; BMI: 36.2 +/- 1.8 and 36 +/- 1.5 respectively) before and after 8 weeks of a 1000 kCal/day diet. The responses of serum TSH and PRL to TRH (200 micrograms iv as a bolus) were evaluated as Area Under the Curve (AUC) and net increase in respect to basal values (delta TSH and delta PRL). Serum T4, fT4 and rT3 were assayed at the baseline and T3 and fT3 at the baseline and 120' after TRH injection. A similar analysis was performed on 14 age- and sex-matched lean subjects as controls. In females at baseline fT4 serum levels were greater than controls and were significantly reduced after weight loss; rT3 increased after weight loss in the whole study group. In patients of both sexes the PRL peak after TRH injection was earlier but not greater (15') than in controls (30'). After weight loss PRL peak after TRH was found at 30' (as controls) in females only. Taking into consideration the stage of pubertal development, the results were the following: a) in puberal girls, after weight loss, TSH and PRL peaks after TRH were delayed with respect to baseline and to the other considered subgroups; b) in prepubertal girls TSH and PRL peaks, delta TSH, delta PRL, AUC-TSH and AUC-PRL were blunted with respect to pubertal ones; c) the other considered variables were unchanged after the period of caloric deprivation. No correlation between BMI and the AUC of TSH and PRL was found. These data suggest that thyroid function is substantially normal in adolescent obese subjects and not influenced by a prolonged period of caloric restriction, even though a reduced hypothalamic dopaminergic tone on pituitary thyreotrophs and lactotrophs could cause subtle alterations on TSH and PRL release, partially influenced by gender and sexual development.

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