Pituitary Regulation of Thyroid Function in Pregnancy

Abstract

Protein bound iodine (PBI), free and protein bound thyroxin (T4) and plasma thyrostimuline levels (TSH) have been determined during the pregnancy of 14 healthy women and of five patients treated for a slight hypothyroidism. In normal pregnant women, PBI, T4 bound to thyroxin binding globulin (TBG) and plasma TSH levels were increased except in two cases, but the dialysable fraction of T4 (free fraction) remained in the lower normal range.

When results were reported according to the weeks of pregnancy, it was shown that the plasma TSH began to increase as early as the sixth week of pregnancy, remained high, except for the period between the 20th and 22nd week, where plasma TSH was normal. After delivery, the plasma TSH level returned to normal range.

In the treated hypothyroid pregnant women, the plasma TSH remained at normal range and the free dialysable fraction of T4 was normal or slightly increased.

These results suggest that the increased TBG capacity observed in pregnancy is responsible for the higher thyroid hormone synthesis necessary to maintain a sufficient free T4 fraction. This increased thyroid hormone supply needs the TSH stimulation; plasma TSH levels were indeed found increased in this study for the group of normal pregnant women, but were found normal when exogenous thyroid hormones were given to the hypothyroid women.

Key words

Pregnancy - Thyrostimuline - Thyroid Function