Free Triiodothyronine (FT₃) — Reference Values

Free T3 levels are generally stable in pregnancy, with mild variation by trimester. Due to assay variability, interpretation should always be made alongside TSH and Free T4.

Units Nonpregnant Adult 1st Trimester 2nd Trimester 3rd Trimester
pg/mL 2.4 – 4.2 4.1 – 4.4 4.0 – 4.2 Not reported
pmol/L 3.7 – 6.5 6.3 – 6.8 6.2 – 6.5 Not reported
Values from Abbassi-Ghanavati et al., Obstet Gynecol 2009.
Physiology of Free T3 in Pregnancy
  • Estrogen increases thyroxine-binding globulin (TBG), but Free T3 typically remains stable.
  • Total T3 increases due to increased protein binding, while Free T3 changes minimally.
  • Placental deiodinases convert maternal T4 to T3 and reverse T3, influencing values.
  • Assessment of thyroid status in pregnancy should rely primarily on TSH and Free T4.
Causes of Elevated Free T3
  • Graves’ disease
  • Toxic multinodular goiter or toxic adenoma
  • Thyroiditis (hyperthyroid phase)
  • TSH-secreting pituitary adenoma
  • Excess thyroid hormone therapy
Causes of Low Free T3
  • Euthyroid sick syndrome (low T3 syndrome)
  • Hypothyroidism (primary or central)
  • Severe illness, caloric restriction, malnutrition
  • Medications reducing peripheral T4→T3 conversion (e.g., amiodarone, steroids)
References
  1. Abbassi-Ghanavati M, Greer LG, Cunningham FG. Pregnancy and laboratory studies: a reference table for clinicians. Obstet Gynecol. 2009;114:1326–1331. PMID 19935037.
  2. Alexander EK et al. 2017 ATA Guidelines for Thyroid Disease in Pregnancy. Thyroid 2017.