Characteristics of infertility and the improvement of fertility by thyroxine treatment in adult male hypothyroid rdw rats

We previously reported that rdw rats were infertile in both sexes. The pres ent study was conducted to determine whether hypothyroidism in adult male r dw rats induced infertility by impairing sexual behavior or testicular func tion, whether the infertility could be reversed by thyroxine (T-4) treatmen t, and whether the mutant could be produced by infertile rdw rats via in vi tro fertilization. The sexual behavior was analyzed by pairing with normal female rats. The fertility of epididymal sperm was determined by in vitro f ertilization. The results indicated that the infertility resulted from both defective sexual behavior and testicular function. No untreated rdw rats m ated. The weights of epididymides were significantly low, whereas those of testes were not different from those of untreated normal rats. Epididymal s perm with cytoplasmic droplets were observed at a significantly high freque ncy. No fertilization was detected either in vivo or in vitro. Thyroxine tr eatment markedly increased serum T-4 levels and the weights of both epididy mides and testes. Partial reversion of the impaired sexual behavior was obs erved, and the percentage of epididymal sperm with cytoplasmic droplets was markedly decreased after T-4 treatment. Fertility of epididymal sperm was completely reversed when determined both in vivo and in vitro, and homozygo us embryos developed to term after transfer without loss of viability.