Maternal Thyroid Function in Early and Late Pregnancy¹

Abstract

Thyroid function was investigated during and after pregnancy in 12 healthy euthyroid women. During pregnancy, serum total T₄ (TT₄) levels were significantly elevated and nearly stable, while thyroxine-binding globulin (TBG) levels progressively increased till the 7th month. A slight elevation, though not significant, of free T₄ (fT₄) was recorded in early pregnancy. In the following months, fT₄, free T₃ (fT₃) and the T₄/TBG ratio progressively diminished, reaching a plateau at the 7th month. Serum TSH levels, measured by an ultrasensitive immunofluorometric assay, were comparable to postpartum values during the first trimester and showed a moderate upward trend with the progression of pregnancy. The evaluation of 24-hour TSH profiles was performed in 5 women during the first trimester of pregnancy. In all women, the circadian rhythm of TSH was present with a normal nocturnal surge, though anticipated in 1 case. In summary (1) during the first trimester of pregnancy, the increased thyroid activity does not seem to be only sustained by pituitary TSH which remains unmodified; the negative correlation between TSH and hCG levels might suggest that hCG also stimulates the gland to increase thyroid hormone output, and the presence of a normal TSH circadian rhythm indicates that the central mechanism of neuroregulation of the pituitary-thyroid axis is preserved in early pregnancy, and (2) in late pregnancy, a marked decrease in free thyroid hormone fractions is accompanied by serum TSH levels still in the normal range, indicating a modification of thyroid homeostasis which might recognize various etiological factors.

Tandem Mass Spectrometry Improves the Accuracy of Free Thyroxine Measurements During Pregnancy

“…TT 4 levels during pregnancy have been well studied both in countries with iodine sufficiency and iodine insufficiency and their values generally plateau during the second and third trimester at about 150% of their prepregnancy values (3,4,6,7,10,11,(37)(38)(39). The magnitude of the increase in this study was similar to that reported in previous studies.…”

Changes in Thyroid Hormones Serum Profiles During Late Pregnancy and Post Lambing in the Awassi Sheep

“…3) which have been observed throughout the last 4 days of pregnancy may indicate an increase in maternal metabolism to provide the necessary physiological requirements for the crucial task of parturition as well as setting the stage for the initiation of lactation. Our results are in accord with previous clinical observations which have been noted in other mammalian species 16,17,18 . Moreover, an increase in estrogen levels during late pregnancy is associated with an increase in total serum T4 levels in ewes 15 .…”

Clinical Significace of Thyrotropin Variability

“…Причем у 20% беременных от мечаются субнормальные значения ТТГ [38,40,80]. Высокий уровень обладающего тиреостимулиру ющим эффектом ХГ [54,76,100] порой определяет повышенные концентрации св. Т4 в первом тримес тре беременности [38,40,80].…”