Effect of high-dose thyrotropic hormone treatment on the cells of the newborn rat thyroid.

Newborn rats were given 5 I.U. thyrotropic hormone (TSH) either in a single dose or in daily 1 I.U. doses for 5 days. The treatment resulted in the activation of only certain cells of the follicular epithelium. This suggests that the thyroid is responsive in newborn age but not all its cells react. The differences in the response to TSH may be due to different timing of receptor maturation, to the phasic nature of response and to the uneven presence of receptors. It is assumed that cells possessing receptors multiply to form the mature tissue.