Paradoxical hypertrophy and plasticity of the testis in rats recovering from early thyroid deficiency: a growth study including effects of age and duration of hypothyroidism.

The effects of various durations of postnatal hypothyroidism followed by recovery were studied on testicular growth in Long-Evans and Sprague-Dawley rats from birth to 7 months. Hypothyroidism was induced by adding propylthiouracil (PTU) in drinking water (0.1%, w/v). Recovery was induced by withdrawal of PTU. Testicular growth was reduced in rat pups by 20, 65 and 90% at days 10, 25 and 50. Upon withdrawal of PTU at weaning (25 days), testicular growth resumed and became compensatory; catch-up growth occurred by day 65. Paradoxically, testicular growth progressively increased, surpassing the control weights by 40, 50 and 100% at days 75, 90 and 210. Maximal testicular growth rate in the recovery group was 35% higher, occurred 2 weeks later and lasted 2 weeks longer than controls. Testes of rats subjected to prolonged postnatal hypothyroidism (60 or 120 days) also showed recovery and hypertrophy, amounting to nearly twice the normal maximal growth levels, after at least 6 months of recovery. Body weights of recovering rats remained always significantly below those of controls. When the suckling pups were exposed to short, week-long regimes of PTU treatment, only rats treated during the early postnatal weeks (days 1-8 or 9-16) had enlarged testes; PTU treatment during the late suckling period (days 17-24) or later had no effects. Total duration of hypothyroidism in the suckling period was positively correlated with testicular enlargement. The results indicate that hypothyroidism early in life is stimulatory to testicular growth, resulting in a paradoxical twofold increase in final testicular size which occurs even if hypothyroidism is prolonged. These effects occur similarly in different strains of rat with differing sized testes. It is suggested that there is a sensitive period for this effect, i.e. during the first 2 weeks after birth. The marked plasticity of testicular growth as shown by its recovery and hypertrophy, even after long periods of hypothyroid retardation, is unique in the body and may be a useful model for studying hormonal factors regulating testicular growth and for animal breeding and research into infertility.