Thyroidal response to peroral TSH in suckling and weaned rats.

The biological activity of perorally administered bovine thyroid-stimulating hormone (bTSH) was investigated in suckling (14-day-old) and weaned (30-day-old) rats. Animals were treated with water or bTSH given either by subcutaneous injection or by the oral route. Both suckling and weaned animals responded to subcutaneous administration of bTSH by an increase in serum thyroxine (T4) and triiodothyronine (T3) concentrations. Bovine TSH administered orally to weaned animals had no effect on thyroid hormone levels. In contrast, suckling pups responded to perorally administered bTSH with increases of T4 and T3 levels comparable to those achieved in animals that received bTSH subcutaneously. The characteristics of the gastric secretion and intestinal epithelium of the weaned animal, therefore, appear to play a role in preventing absorption of a polypeptide hormone. However, protein hormones contained in maternal milk can be transferred to the suckling in which their full biological functions can be expressed.