Is total thyroxine better than free thyroxine during pregnancy?

Area of Science:

• Endocrinology
• Obstetrics
• Perinatology

Background:

• Thyroid dysfunction during pregnancy poses risks to both mother and fetus.
• Establishing gestational-age specific reference ranges for thyroid hormones is crucial for accurate diagnosis.
• Subclinical hypothyroidism (SCH) is defined by elevated thyroid-stimulating hormone (TSH) with normal thyroxine levels.

Purpose of the Study:

• To establish a gestational-age specific reference curve for serum total thyroxine (T4) levels.
• To compare pregnancy outcomes between euthyroid women and those with SCH.
• To evaluate the diagnostic utility of total T4 versus free T4 in identifying SCH.

Main Methods:

• Serum thyroid analytes were measured in 17,298 pregnant women over 2.5 years.
• Gestational-age specific quantile curves for total T4 were developed for women in the first 20 weeks of gestation.
• Pregnancy outcomes were compared between euthyroid women and those with SCH (elevated TSH with normal total T4 or free T4).

Main Results:

• Serum total T4 levels increased into the second trimester, plateauing around 16 weeks.
• Established thresholds for total T4 ranged from 5.3–8.0 μg/dL (lower) to 12.6–16.4 μg/dL (upper).
• Women with SCH (defined by free T4, total T4, or both) had a significantly higher risk of preterm delivery (P = .007) and placental abruption (P = .013) compared to euthyroid women.

Conclusions:

• Both free T4 and total T4 determinations are equally effective in identifying SCH when TSH is elevated.
• Women with SCH are at increased risk for adverse pregnancy outcomes, including preterm birth and placental abruption.
• Accurate identification of SCH through thyroid hormone monitoring is vital for managing high-risk pregnancies.