Effect of thyroxine replacement therapy on the growth patterns of body, brain, and cerebellum in the neonatal hypothyroid rat.

The reversibility of the effects of propylthiouracil induced neonatal hypothyroidism by daily subcutaneous injections of L-thyroxine (T4) was investigated in the rat. Four groups of rat pups were used in this experiment: Group I (control) received equivalent amounts of saline subcutaneously; Group II (hypothyroid) received propylthiouracil injections daily; Group III (T4-14) are hypothyroid rats who received daily T4 replacement therapy (2.5 micrograms/day) subcutaneously starting on Day 14; Group IV (T4-21) are also hypothyroid rats whose T4 replacement therapy (2.5 micrograms/day) was started on Day 21. The four parameters studied were body length, body weight, brain weight, and cerebellar weight in these four groups of animals at 4, 7, 10, 14, 21, 28, 35, and 42 postnatal days. A minimum of eight animals were studied for each time sequence. The present study clearly indicated the beneficial effect of T4 therapy on the length and body weight of the hypothyroid animals, whether therapy was started at Postnatal Day 14 or 21. In addition, a more important finding of this study was the significant increase in brain and cerebellar weights of T4-treated animals compared to the hypothyroid animals, this beneficial effect being more striking in the T4-14 group when compared to that in the T4-14 animals.