Triiodothyronine, Total (T₃) — Reference Values

Total T3 rises during pregnancy due to increased estrogen-stimulated thyroxine-binding globulin (TBG). Free T3 stays more stable, while total T3 increases progressively across trimesters.

Units Nonpregnant Adult 1st Trimester 2nd Trimester 3rd Trimester
ng/dL 77–135 97–149 117–169 123–162
nmol/L 1.19–2.08 1.49–2.29 1.8–2.6 1.89–2.29
Values from Abbassi-Ghanavati et al., Obstet Gynecol 2009.
Physiology of Total T3 in Pregnancy
  • Estrogen increases hepatic production of TBG → increased total T3 concentrations.
  • Total T3 may nearly double by late pregnancy, paralleling increase in total T4.
  • Free T3 remains relatively stable despite total T3 elevation.
  • Placental deiodinase activity increases conversion of T4 to T3 and reverse T3.
  • Thyroid assessment in pregnancy should rely on trimester-specific TSH and Free T4.
Causes of Elevated Total T3
  • Graves’ disease
  • Toxic adenoma or toxic multinodular goiter
  • Thyroiditis (hyperthyroid phase)
  • Excess thyroid replacement therapy
  • High TBG states (pregnancy, estrogen therapy) — total T3 rises but Free T3 is normal
Causes of Low Total T3
  • Primary or secondary hypothyroidism
  • Euthyroid sick syndrome (low T3 pattern)
  • Severe illness or malnutrition
  • Medications: β-blockers, amiodarone, glucocorticoids
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. Kratz A, Ferraro M, Sluss PM, Lewandrowski KB. Laboratory reference values. N Engl J Med. 2004;351:1548–1563. PMID 15470219.