Growth and Function of the Thyroid Gland in Rats Injected with L-Thyroxine from Birth to Maturity

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Abstract
L-Thyroxine was injected daily in growing rats from the second to fourth day after birth until maturity, 90–115 days, at doses which maximally suppressed TSH secretion. The thyroid glands of these rats grew, despite suppressed TSH secretion. After 115 days, their thyroid weights, cell heights and I131 uptakes were comparable to those of adult litter mates which had been treated with L-thyroxine for only 3 weeks (21 days) before killing. In litter mates injected with thyroxine for 95 days, thyroid weight, cell height and I131 uptake, 3 weeks later, were comparable to those of controls which had never received exogenous thyroxine. Although the prolonged thyroxine treatment thus did not prevent a rapid recovery of normal structure and function, it did delay the recovery of normal potential for augmented growth. The response of the treated rats to PTU was clearly less than that of their litter mate controls. This reduction in growth potential disappeared eventually. Similarly treated rats responded, 2½ months after thyroxine treatment was discontinued, with full-fledged goiter development.

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