At what week are serum thyroxine (T4) and Thyroid-Stimulating Hormone (TSH) levels expected to normalize in pregnancy?

From the Guidelines

Serum thyroxine (T4) and Thyroid-Stimulating Hormone (TSH) levels are expected to normalize by 16-20 weeks of pregnancy. During early pregnancy, human chorionic gonadotropin (hCG) levels rise rapidly, peaking around 8-10 weeks gestation, which can stimulate the thyroid gland, causing a temporary increase in T4 and suppression of TSH 1. As pregnancy progresses and hCG levels decline after the first trimester, these thyroid parameters gradually return to their normal ranges. This normalization occurs as the fetal thyroid gland begins to function around 12 weeks and becomes fully functional by 18-20 weeks. Understanding this timeline is important for clinicians monitoring thyroid function during pregnancy, as abnormal thyroid function beyond this period may indicate true thyroid dysfunction requiring treatment rather than normal physiological adaptation to pregnancy. Key points to consider include:

• The importance of monitoring serum TSH concentration every 6 to 8 weeks during pregnancy in treated hypothyroid women, as the requirement for levothyroxine frequently increases during pregnancy 1
• The potential benefits of levothyroxine therapy in pregnant women with elevated serum TSH, including reduced risk of fetal wastage or subsequent neuropsychological complications in the offspring due to thyroid insufficiency 1.

From the FDA Drug Label

In pregnant patients with primary hypothyroidism, maintain serum TSH in the trimester-specific reference range Monitor TSH every 4 weeks until a stable dose is reached and serum TSH is within normal trimester-specific range.

The FDA drug label does not answer the question.

From the Research

Serum Thyroxine and TSH Normalization in Pregnancy

• The normalization of serum thyroxine (T4) and Thyroid-Stimulating Hormone (TSH) levels in pregnancy is a complex process that varies throughout the gestational period 2, 3, 4, 5, 6.
• Studies have shown that TSH levels decrease in the first trimester and increase in the second and third trimesters, but not to pre-pregnancy levels 4.
• The reference intervals for TSH, free thyroxine (FT4), and free triiodothyronine (FT3) vary by trimester, with the first trimester having lower TSH levels and higher FT4 and FT3 levels compared to the second and third trimesters 3, 6.
• The exact week at which serum T4 and TSH levels are expected to normalize in pregnancy is not explicitly stated in the available evidence, but it is known that the thyroid function tests change throughout the gestational period 2, 3, 4, 5, 6.
• The American and European thyroid associations recommend the use of local trimester-specific reference intervals for thyroid function tests during pregnancy 6.
• The reference intervals for TSH, FT4, and FT3 in the first trimester of pregnancy have been established in several studies, with varying results depending on the population and location 3, 5, 6.
• For example, one study found that the first trimester-specific reference intervals for TSH, FT4, and FT3 were 0.20-4.60 mIU/L, 9.0-18.02 pmol/L, and 3.40-5.64 pmol/L, respectively 6.
• Another study found that the reference intervals for TSH, FT4, and FT3 in the first trimester were 0.49-2.33 mIU/L, 10.30-18.11 pmol/L, and 3.80-5.81 pmol/L, respectively 3.