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Regulation of Maternal Thyroid during Pregnancy^*

Departments of Endocrinology (D.G.), Nuclear Medicine (P.B., J.K.), Gynecology and Obstetrics (C.R., B.L.), Radiology (M.L.), Saint Pierre Hospital, Universitè Libre de Bruxelles Brussels
The Center for Reproductive Medicine, Academic Hospital, Vrije Universiteit Brussel (A.V.S.) Brussels
The Department of Nuclear Medicine, Clinique Saint-Luc, Universitè Catholique de Louvain (P.D.N.) Woluwe, Belgium

Address requests for reprints to: Dr. Daniel Glinoer, Laboratory of Radioisotopes, Hospital Saint Pierre, Rue Haute 322, B-1000 Brussels, Belgium.

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 µg/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.

^* This work was supported by Henning (Berlin) and Triosol (Brussels). Presented in part at the 8th Joint Meeting of the British Endocrine Societies (Manchester, England, April 1989), the 71st Annual Meeting of The Endocrine Society (Seattle, WA, June 1989), the 18th Annual Meeting of the European Thyroid Association (Copenhagen, Denmark, June 1989), the 64th Annual Meeting of the American Thyroid Association (San Francisco, CA, September 1989).

Received December 5, 1989.

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