The correct answer is:

In a healthy patient, thyroid-stimulating hormone (TSH) levels decrease in the first trimester and then return to normal in the second and third trimester.

Educational Objective:

Discuss the process of diagnosing hypothyroidism during pregnancy.

Key Point:

Thyroid stimulating hormone (TSH), thyroxine (T4), and triiodothyronine (T3) concentrations in pregnancy are different from those in non-pregnant adults and vary by trimester. Due to these physiologic variations, pregnancy- and trimester-specific reference intervals for TSH should be used.

Explanation:

Pregnancy normally alters several components of the normal thyroid panel: 

1. There is a physiological and transient reduction in TSH seen usually between 8 and 14 weeks of gestation. This is due to the thyroid stimulating effects of the physiological increase in hCG early in pregnancy. In around 20% of women, the TSH may even drop below the normal range. After 14 weeks, the TSH usually returns to normal.
2. Due to the increased circulating estrogen levels, there is an increase in thyroid binding globulin (TBG) that begins during the first half of pregnancy and continues to term.
3. The increase in TBG results in lower levels of free T3 and T4 which stimulates the hypothalamic-pituitary-thyroid axis, which…
4. ...results in higher levels of total (not free) T3 and T4.
5. In addition, the abundance of deiodinase enzymes found in the placenta result in an increase in the metabolism of T4 to T3.
   

This free T4 is an unreliable measure of thyroid status in pregnancy.

Even short-term hypothyroidism can pose a risk to this patient; therefore, she should receive appropriate treatment if her labs indicated hypothyroidism. Most labs will have their own, specific pregnancy-adjusted reference intervals. If such pregnancy-specific reference intervals are not available, then those published by various societies can be used.

The American Thyroid Association (ATA)

The American College of Obstetricians and Gynecologists (ACOG)

The Endocrine Society (ES)

References:

Nader S. Thyroid Disease and Pregnancy. Book Chapter in Creasy and Resnik's Maternal-Fetal Medicine: Principles and Practice, 60, 1022-1037.e4

Alexander EK, Pearce EN, Brent GA, Brown RS, Chen H, Dosiou C, Grobman WA, Laurberg P, Lazarus JH, Mandel SJ, Peeters RP, Sullivan S. “2017 Guidelines of the American Thyroid Association for the diagnosis and management of thyroid disease during pregnancy and the postpartum.” Thyroid 2017, 27(3):315-389.

Vaidya B, Hubalewska-Dydejczyk A, Laurberg P, Negro R, Vermiglio F, Poppe K. Treatment and screening of hypothyroidism in pregnancy: results of a European survey. Eur J Endocrinol. 2012;166(1):49-54.