Thyrotropic hormone and thyroidal function initiation in fetal rat.

In rat fetus thyrotropic hormone (TSH) is present in the blood as early as day 16 and reaches maximum on day 18. On day 21, TSH level falls off to an intermediate value. Thyroxine is detected in the plasma of fetuses from day 18; the subsequent increase in T4 concentration until day 21, probably generates the negative feedback controlling TSH secretion. The observed increase and maximum in plasma TSH are concomitant with the highest concentrations of cAMP in the thyroid. There may be a causal relationship behind this concomitance since TSH stimulates in vitro the accumulation of cAMP in explanted thyroid from 17 days-old fetuses. The implications of these findings on the role of TSH in the developing rat thyroid are discussed.