Congenital hypothyroid mutant male
rdw rats have enlarged testes in adulthood with dwarfism accompanied by infertility. To explain how
rdw rats acquire enlarged testes in adulthood, we compared age-matched normal (N) rats at various developmental stages for blood levels of hormones, thyroxine (T4), follicle stimulating hormone (FSH), luteinizing hormone (LH) and testosterone (T), and investigated whether T4 therapy (
rdw+T4) from 3 weeks of age (w) until adulthood could induce recovery of fertility in
rdw rats, as well as how
rdw+T4 affected hormonal patterns. Testes weights of
rdw rats were higher than those of N rats at 19 w in adulthood though it was low during development. Serum T4 values in
rdw rats were markedly lower than those in N rats but steadily increased up to 19 w. The serum FSH values in
rdw rats were lower than those in N rats at all ages, and neither serum LH nor T value was significantly different at any age. The testes weight of
rdw+T4 rats was significantly higher than that of N rats at 13 w with recovered growth, and was higher than that of
rdw rats at 19 w. When they were mated with proestrous females after 16 w, all females became pregnant and gave birth to a normal number of pups. The T4 and FSH values of
rdw+T4 rats were significantly higher than those in
rdw rats, but similar to those in N rats in adulthood. The results suggest that even low levels of circulating thyroid hormone (TH) in
rdw rats stimulate the development of their testes, probably through Sertoli cells, resulting in the enlarged adult testes without fertility, and that a sufficient circulating TH level from the immature stage plays a pivotal role in restoring mating activity, probably through FSH-mediated action towards adulthood.
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