Late effects of neonatal undernutrition and overnutrition on pituitary-thyroidal and gonadal function.

Neonatal rats were underfed or overfed and then, at weaning, placed on a normal diet and allowed to mature. Although overfed rats were obese at weaning, they were of normal weight when mature. The underfed rats remained smaller all their lives. The endocrine organs did not share in this retardation, and were actually larger in several instances when expressed per 100 g body weight. Underfed rats also had delayed eye-opening, normal puberty and normal estrous cycles. Pituitary, hypothalamic and serum TSH concentrations were normal, or even increased (in the underfed females). The responses to propylthiouracil and to thyrotropin-releasing hormone (TRH) were both normal. Thus the tertiary hypothyroidism reported in underfed rats 16 days old did not persist into adult life. In all comparisons the males had higher serum TSH concentrations than females and usually had a greater response to TRH stimulation. The various persistent endocrine abnormalities resulting from neonatal thyrotoxicosis differ from those seen after neonatal caloric deprivation in all pertinent regards except that both have reduced body weights.