Parameters of thyroid function throughout and after pregnancy in an iodine-deficient population

The thyroid hormone milieu is of crucial importance for the developing fetus. Pregnancy induces physiological changes in thyroid homeostasis that are influenced by the iodine status. However, longitudinal studies addressing thyroid function during pregnancy and after delivery are still lacking in mild-to-moderate iodine-deficient populations. Here we characterize the serum parameters of thyroid function throughout pregnancy, and until 1 year after delivery, in a population of pregnant women whom we have previously reported to be iodine deficient (median urinary iodine levels below 75 microg/L).

One hundred eighteen pregnant women were studied. Clinical data were recorded and serum was collected. Serum total and free thyroxine (T(4)) and triiodothyronine (T(3)), thyroid-stimulating hormone, thyroxine-binding globulin, and thyroglobulin were measured.

Mean total T(4) ranged from 159 at the start of gestation to 127 nmol/L at 1 year after delivery, free T(4) from 14.2 to 17.8 pmol/L, total T(3) from 2.4 to 2.1 nmol/L, free T(3) from 6.7 pmol/L to 6.4 pmol/L, thyroid-stimulating hormone from 1.2 to 1.4 mIU/L, T(4)-binding globulin from 62.0 to 26.9 mg/L, and thyroglobulin from 11 to 10 microg/L.

The pregnant women in this study had an absence of the usual free T(4) spike and a smaller than expected increment in total T(4), described during pregnancy in iodine-sufficient populations. A greater number of women had subclinical hypothyroidism compared with iodine-sufficient populations. This hormonal profile, most likely due to iodine insufficiency, may result in inadequate thyroid hormone supply to the developing fetus. We conclude that care should be taken when reviewing the results of thyroid hormone tests in iodine-insufficient populations and when no gestation-specific reference values have been established. In addition, we recommend iodine supplementation in our population and populations with similar iodine status, particularly during pregnancy and lactation.