During pregnancy, which statement best describes thyroid hormone changes?

During pregnancy, which statement best describes thyroid hormone changes?

• A. Total T4 and T3 increase and peak by 10-15 weeks; Free T4 and T3 decrease in 2nd/3rd trimesters
• B. All thyroid hormones decrease
• C. Free T4 and T3 increase in 2nd/3rd trimesters
• D. No change in thyroid hormones

Answer: (A)

Estrogen during pregnancy increases thyroid-binding globulin (TBG), which raises the amount of thyroid hormone tied up in binding and thus elevates total T4 and T3. The body generally keeps free (unbound) T4 and T3 within a narrow range, but as TBG rises and physiologic changes continue, the free fractions can appear lower in the 2nd and 3rd trimesters, even though overall thyroid activity is adequate. This pattern—rising total T4 and T3 with a relative decline in free T4 and T3 later in pregnancy—is what the statement captures. In practice, interpreting thyroid function in pregnancy relies more on free T4 and TSH than on total T4/T3 because total levels are shaped by binding proteins that change with pregnancy.

Estrogen during pregnancy increases thyroid-binding globulin (TBG), which raises the amount of thyroid hormone tied up in binding and thus elevates total T4 and T3. The body generally keeps free (unbound) T4 and T3 within a narrow range, but as TBG rises and physiologic changes continue, the free fractions can appear lower in the 2nd and 3rd trimesters, even though overall thyroid activity is adequate. This pattern—rising total T4 and T3 with a relative decline in free T4 and T3 later in pregnancy—is what the statement captures. In practice, interpreting thyroid function in pregnancy relies more on free T4 and TSH than on total T4/T3 because total levels are shaped by binding proteins that change with pregnancy.