Reference Values for Free Thyroxine (Free T4)
Free Thyroxine (Free T4) — Trimester-Specific Reference Ranges
Free T4 decreases slightly during pregnancy due to rising thyroxine-binding globulin (TBG). Use trimester-specific reference ranges for accurate interpretation.
| Units | Nonpregnant | 1st Trimester | 2nd Trimester | 3rd Trimester |
|---|---|---|---|---|
| ng/dL | 0.8 – 1.7 | 0.8 – 1.2 | 0.6 – 1.0 | 0.5 – 0.8 |
| pmol/L | 10.3 – 21.9 | 10.3 – 15.5 | 7.7 – 12.9 | 6.4 – 10.3 |
Physiologic changes in pregnancy
- Estrogen increases TBG → reduces measurable Free T4.
- Total T4 rises, but Free T4 remains normal or slightly lower.
- Use trimester-specific Free T4 and TSH together for interpretation.
Causes of high Free T4
- Hyperthyroidism (Graves’ disease most common in pregnancy).
- Thyroiditis (subacute, painless, or postpartum thyroiditis).
- Toxic multinodular goiter or autonomous thyroid nodules.
- Excess thyroid hormone replacement.
- hCG-mediated hyperthyroidism (hyperemesis, molar pregnancy).
Causes of low Free T4
- Primary hypothyroidism (Hashimoto’s thyroiditis).
- Inadequate thyroid hormone replacement.
- Secondary hypothyroidism (pituitary disease → low TSH & low FT4).
- TBG excess falsely lowering apparent FT4 in some assays.
- Severe systemic illness (non-thyroidal illness syndrome).
References
- Abbassi-Ghanavati M, Greer LG, Cunningham FG. Pregnancy and laboratory studies: a reference table for clinicians. Obstet Gynecol. 2009;114:1326–1331.
- Alexander EK, et al. 2017 Guidelines of the American Thyroid Association. Thyroid. 2017;27:315–389.
- Perinatology.com — Reference Values for Free Thyroxine (Free T4).