Free T4 immunoassays are flawed during pregnancy

Area of Science:

• Endocrinology
• Reproductive Medicine
• Clinical Chemistry

Background:

• Thyroid hormone levels undergo significant physiological changes during pregnancy.
• Accurate assessment of thyroid function is crucial for maternal and fetal health.
• Existing free thyroxine (FT4) immunoassays may not accurately reflect these gestational changes.

Purpose of the Study:

• To evaluate the diagnostic accuracy of two commonly used free thyroxine (FT4) immunoassays in pregnant women.
• To compare the performance of FT4 immunoassays with total T4 (TT4) and free T4 index (FT4I) during pregnancy.

Main Methods:

• Serum samples were collected from healthy, thyroid peroxidase antibody-negative women during each trimester of pregnancy and from nonpregnant controls.
• Measurements included thyrotropin (TSH), TT4, FT4I, and two different FT4 immunoassays.

Main Results:

• Total T4 (TT4) levels were significantly elevated across all trimesters compared to controls.
• Free T4 index (FT4I) was elevated in the first trimester but returned to nonpregnant levels by the second trimester.
• Both FT4 immunoassays showed values comparable to or lower than controls in the first trimester, decreasing further in later trimesters.

Conclusions:

• Neither of the evaluated FT4 immunoassays accurately reflects the known changes in free T4 during pregnancy.
• Total T4 (TT4) and the FT4I demonstrated a reliable inverse relationship with TSH throughout pregnancy.
• TT4 and FT4I appear to be more dependable estimates of free T4 status during pregnancy compared to the studied immunoassays.