Pregnancy changes the thyroid axis in several ways at once, so both the normal ranges and the thyroid hormone requirement differ from the non-pregnant state.

Thyroid function in pregnancy

Five factors act together.

hCG. A transient increase of human chorionic gonadotropin (hCG) occurs in the first trimester. hCG is similar enough to thyroid-stimulating hormone (TSH) that it slightly activates the TSH receptor, TSH-R, so the gland is stimulated a little. In some women this leads to transient gestational hyperthyroidism, characterised by hyperemesis gravidarum, a condition with severe nausea and vomiting and a high risk of volume depletion.
Thyroxine-binding globulin (TBG). Estrogen raises TBG during the first trimester by increasing its sialylation, which lengthens its half-life in the blood.
Immune system. An alteration in the immune system can lead to the exacerbation or formation of autoimmune thyroid diseases.
Placental deiodinase. Placental type III thyroid deiodinase increases thyroid hormone metabolism.
Iodine. Iodine excretion in the urine increases.

Effect on hormone levels and normal ranges

The effect of hCG during the first trimester on the secretion of TSH is inhibitory to some extent, so the normal ranges differ between the trimesters of gestation: during the first trimester TSH decreases, and from mid-gestation it returns to the normal non-pregnant level.

Total triiodothyronine (T3) and thyroxine (T4) levels are 1.5 times the normal values during pregnancy. T4 during the first trimester is normal or even higher, and it decreases toward the normal lower cut-off at the end of the third trimester.

Two lines across three trimester bands showing TSH dipping in the first trimester and total T4 staying at 1.5 times normal, then falling near the end of the third trimester. — TSH dips early in pregnancy, while total T4 stays elevated and falls late in the third trimester.

Effect on hormone and iodine requirement

Together these changes raise the demand for hormone and for iodine. The WHO therefore suggests that all pregnant women, and women during the breastfeeding period, have 250 micrograms per day of iodine in their supplemental diet.

The thyroid hormone requirement for hypothyroid patients who use levothyroxine during pregnancy increases by up to 45% of their normal dosage.