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Laboratório de Endocrinologia Molecular, Instituto de Biofísica Carlos Chagas Filho, CCS-BlocoG-Cidade Universitária, IIha do Fundão, 21949.900 Rio de Janeiro, Brazil

(Requests for offprints should be addressed to C C Pazos de Moura; Email: maveiga{at}biof.ufrj.br)

We investigated the influence of hypo- and hyperthyroidism on the ability of leptin to modulate TSH secretion. Two hours after receiving leptin (8 µg leptin/100 g BW; s.c.), hyperthyroid rats (10 µg thyroxine (T4)/100 g body weight (BW) for 5 days) showed a 1.7-fold increase in serum TSH (P<0.05); in hypothyroid rats, leptin had no effect. Hemi-pituitaries of hyperthyroid rats incubated with 10^–9 and 10^–7M leptin showed reductions in TSH release of 40 and 50% respectively (P<0.05); incubation with 1:2000 and 1:500 dilutions of antiserum against leptin resulted in 3- and 4-fold higher TSH release (P<0.05 and P<0.001 respectively). However, in hypothyroid pituitaries leptin or the antiserum had no effect. The results suggest that the in vivo and in vitro responsiveness of TSH to leptin is abolished in hypothyroidism and is preserved in short-term hyperthyroidism, in comparison to previous reports in euthyroidism. In addition, the inhibitory action of pituitary leptin is enhanced in hyperthyroid glands, which may suggest a role for locally produced leptin in the suppression of TSH release associated with hyperthyroidism.

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