Hormonal Control of Growth in the Infant Rat: Further Evidence that Neither Thyrotropin Nor Thyroid Hormones Are Transferred Via Milk to Suckling Pups*

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Abstract
Experiments were conducted to obtain information on the possible transfer of TSH and thyroid hormones from mother rats to their infants via milk. The mothers were thyroidectomized by surgical or chemical (tapazole) means to eliminate maternal thyroid hormones from milk while increasing its content of TSH. Their pups were either hypophysectomized (HX) or sham HX, and some pups were also chemically thyroidectomized (CTX) by tapazole injections. Surgical thyroidectomy (STX) of the mothers reduced the weight gain of HX and sham HX pups by 28% and 22%, respectively, but it did not affect tail growth in either group of pups, and it did not affect serum T4 levels in the sham HX infants. In contrast, CTX caused a pronounced inhibition of weight gain (–45%), tail growth (–45%), and serum T4 (to <0.5 μg/100 ml) in the sham HX pups. In comparison to the HX pups with euthyroid or STX mothers,the HX infants with CTX mothers also showed reduced weight gain (–57% and –40%, respectively) and tail growth (–40% and –34%, respectively). Tapazole injections into sham HX pups suckling CTX mothers further reduced weight gain, but not tail growth. Because skeletal growth in the pups is highly dependent on their thyroid status, the lack of effect of surgical TX of mothers on tail growth in their sham HX infants provides no evidence for the transfer of physiologically effective amounts of thyroid hormones from mother to infant via milk. Likewise, the failure of TX of mothers to affect the tail growth of the HX pups argues against milk TSH being of significance to the infant. However, the data obtained with the tapazole-treated mothers indicate that significant quantities of the drug are passed from mother to infant via milk.