Neonatal hypothyroidism: effect on the circadian variation of serum TSH in adult male rats

: Newborn rats were rendered hypothyroid during the first post-natal week and then studied as adults. At 80 days of age, groups of control and neonatally treated animals were sacrificed at the peak (13.00 h) and trough (07.00 h) of the serum thyroid stimulating hormone (TSH) circadian rhythm. Both control and treated animals showed the normal peak and trough fluctuation of serum TSH, although the animals made hypothyroid neonatally had serum TSH levels which were less than those of the control animals. Serum thyroxine (T4) levels of control and treated animals were similar at the trough of the serum TSH rhythm (07.00 h), but the (T4) levels of the neonatally hypothyroid animals were significantly lower than those found in the control animals at the peak of the TSH rhythm (13.00 h). The results of this experiment indicate that the central nervous system (CNS) regulation of the circadian pattern of serum TSH is maintained in adult animals made hypothyroid neonatally, and supports the results of previous studies which indicate an enhanced sensitivity of TSH secretion to feedback suppression by thyroid hormones.