Effects of thyroid hormones on the ontogeny of oestradiol-binding sites in the rat.

The effects of neonatal hypothyroidism on the development of cytoplasmic oestradiol-binding sites were studied in the hypothalamus, pituitary and uterus of the rat. Between 7 and 28 days the total number of binding sites was significantly reduced in all three tissues, whereas the actual concentrations of binding sites were dramatically depressed throughout the period considered in the pituitary only. Adequate replacement therapy by L-thyroxine almost completely reversed all these effects in 14-day-old rats, and administration of excessive L-thyroxine even raised pituitary binding-site concentrations to above normal. On the other hand, plasma oestrogen levels, which were depressed in hypothyroid animals, fell even further, whatever the L-thyroxine therapy. Hence, thyroid hormones appear to stimulate oestradiol binding-site levels directly rather than indirectly via plasma oestrogen titres. The specific modulatory effect of L-thyroxine on oestradiol-binding sites in various types of pituitary cells is discussed.