Characteristics of infertility in female hypothyroid (hyt) mice

Female hypothyroid (hyt) mice are infertile, but the reason for this infertility is not yet known. The present study was conducted to determine whether hypothyroidism induced infertility in immature and mature hyt mice. Furthermore, animals were treated with thyroxine and gonadotrophins at different times to determine whether infertility was due to failure of follicular development, implantation or pregnancy. There were no significant differences in the numbers of ovulated eggs induced by gonadotrophin treatment or the percentages of eggs developed in vitro among immature normal controls, hyt and thyroxine-treated hyt mice. Mature hyt mice showed continuous dioestrus, and ovulated significantly fewer eggs after gonadotrophin treatment and failed to establish pregnancy after mating compared with mature control mice. Mature hyt mice had significantly fewer corpora lutea > 500 microm in diameter and significantly lower progesterone concentrations. Thyroxine treatment before mating in hyt mice resulted in well-developed corpora lutea, an increase in progesterone and normal pregnancy, regardless of subsequent thyroxine administration. In conclusion, infertility occurs in mature rather than immature hyt mice, is due to the failure of follicular development and pregnancy, and can be reversed by thyroxine treatment before mating.