Improvement of follicular development rather than gonadotrophin secretion by thyroxine treatment in infertile immature hypothyroid rdw rats

Despite extensive study of reproductive abnormalities in female hypothyroid animals, little is known of folliculogenesis and gonadotrophin secretion i n spontaneously hypothyroid animals, especially in response to exogenous ho rmone treatment. In this study, follicular development and plasma hormone c oncentrations in the presence or absence of thyroxine and eCG treatment wer e investigated in infertile immature spontaneously hypothyroid rdw rats. Ad ministration of thyroxine once a day from day 21 to day 29 after birth resu lted in increases in body weight (P < 0.001) and ovary mass on day 30 (P < 0.01). Similar populations of both healthy and atretic antral follicles ran ging from 101 to 400 mu m in diameter were observed in control rdw and norm al rats. In rdw rats, thyroxine treatment markedly increased the number of healthy antral uniovular follicles 101-400 or > 550 mu m in diameter in the absence or presence of eCG, respectively. Combined treatment of thyroxine and eCG in rdw rats also markedly increased the number of healthy antral bi ovular follicles. Thyroxine treatment did not affect the population of atre tic antral follicles, but resulted in decrease in the number of atretic lar ge antral follicles (> 400 mu m) in the presence of eCG. Plasma oestradiol concentrations in rdw rats given both thyroxine and eCG were significantly higher than they were in rdw rats given eCG alone (P < 0.001). There were n o significant differences in plasma FSH concentrations on day 28 between rd w (10.7 +/- 1.6 ng ml-l) and normal rats (12.0 +/- 1.4 ng ml(-1); P > 0.05) . Although there were no significant differences in plasma LH concentration s between control rdw (1.9 +/- 0.1 ng ml(-1)) and normal rats on day 30 (1. 8 +/- 0.1 ng ml(-1); P > 0.05), eCG treatment increased plasma LH to a peak concentration 52 h after injection in normal (24.9 +/- 2.4 ng ml(-1)) but not in rdw rats treated with thyroxine (4.8 +/- 0.3 ng ml(-1); P < 0.05). L n conclusion, the results of the present study indicate that thyroxine trea tment improves follicular development but does not rescue the defect of the preovulatory surge of LH in eCG-primed rdw rats.