Regulation of Maternal Thyroid during Pregnancy*

Abstract

A prospective study was undertaken in 606 healthy women during pregnancy to evaluate the changes occurring in maternal thyroid economy as a result of 1) the increased thyroid hormone-binding capacity of serum, 2) the effects of increased levels of hCG on TSH and on the thyroid, and 3) a marginally low iodine intake in the population (50-75 micrograms/day). Four main features were observed. First, thyroidal activity adjusted to the marked increase in serum T4-binding globulin: pregnancy was accompanied by an overall reduction in the T4/T4-binding globulin ratio, with lower free T4 and T3 levels, although in most cases free hormone levels remained within the normal range. The adjustment of thyroidal output of T4 and T3 did not occur similarly in all subjects. In approximately one third of the women, there was relative hypothyroxinemia, higher T3/T4 ratios (presumably indicating preferential T3 secretion), and higher, although normal, serum TSH concentrations. Second, high hCG levels were associated with thyroid stimulation, both functionally (lower serum TSH) and anatomically (increased thyroid size). The data are consistent with a TSH-like effect of hCG on the thyroid. Hence, regulation of the maternal thyroid is complex, resulting from both elevated hCG (mainly in the first half of gestation) and increasing TSH (mainly in the second half of gestation). Third, a significant increase in serum thyroglobulin levels was observed throughout gestation, especially during the last trimester. Fourth, increased thyroid volume was common, and goiter formation not uncommon (goiter was found in 9% of women at delivery). In conclusion, the alterations in maternal thyroid function during gestation are intricate and far from fully understood. In areas of marginally low iodine intake, gestation is associated in a significant number of women with relative hypothyroxinemia, increased thyroglobulin, and enlarged thyroid.

Autoimmune Thyroid Disease in Pregnancy: A Review

“…4 hCG has a much researched thyroid-stimulating hormone (TSH)-like activity secondary to specificity crossover at the TSH receptor (TSHR). As a result, serum thyroxine (T 4 ) and triiodothyronine (T 3 ) levels are elevated, whereas serum TSH levels are reduced.…”

Thyroid stimulating hormone levels rise after assisted reproductive technology

“…However, healthy women do not become overtly hyperthyroid due to compensatory mechanisms involving the hypothalamicpituitary-thyroid axis as well as increased thyroid binding globulin (TBG) levels [30]. In fact, this increased TBG [11] not only counteracts the effects of HCG but actually stimulates a rise in TSH levels that reach a maximum in the second trimester [12]. Given the importance of thyroid function during pregnancy, TSH levels should be maintained below 2.5 mIU/L in order to control for these changes [8].…”

Should human chorionic gonadotropine treatment increase thyroid volume?

“…Many investigations have detected that hCG and TSH share not only structural homology but also their receptors show structural similarity as well, and this homology suggests the basis for the reactivity of hCG with the TSH receptor (9,10). Therefore, high hCG concentrations are associated with thyroid stimulation, both functionally (lower serum TSH concentrations) and anatomically (increased thyroid volume) (11).…”