Difference Between T3 and T4
T4 (thyroxine) is the primary hormone secreted by the thyroid gland and serves as a prohormone that is converted to T3 (triiodothyronine), the metabolically active form that exerts most thyroid hormone effects on target tissues.
Structural and Chemical Differences
T4 contains four iodine atoms and is formed by coupling two molecules of diiodotyrosine (DIT), while T3 contains three iodine atoms and forms from coupling one DIT molecule with one monoiodotyrosine (MIT) molecule 1, 2.
Production and Metabolism
- T4 is the predominant thyroid hormone produced by the thyroid gland, with a 10-20:1 ratio compared to T3 in thyroglobulin 2
- Approximately 80% of circulating T3 comes from peripheral conversion of T4 through deiodination by type I and II deiodinases in tissues like the liver, kidney, and other organs 2, 3
- Only about 20% of T3 is directly secreted by the thyroid gland itself
Protein Binding and Bioavailability
T4 is more tightly protein-bound than T3:
- T4: 99.97% protein-bound to TBG (thyroxine-binding globulin), transthyretin, and albumin 4
- T3: 99.7% protein-bound (slightly less than T4) 4
This difference is clinically significant because T3 enters peripheral tissues more readily due to its lower protein binding, allowing faster access to nuclear receptors 1.
Half-Life and Duration of Action
The elimination half-lives differ substantially:
This explains why T4 provides more stable, sustained thyroid hormone levels, while T3 has more rapid but shorter-lived effects.
Biological Potency and Activity
T3 is approximately 4 times more biologically potent than T4 2. In clinical equivalence, 25 mcg of T3 equals approximately 100 mcg (0.1 mg) of T4 1.
T3 is the metabolically active hormone that binds to nuclear thyroid receptors to control DNA transcription and protein synthesis, while T4 functions primarily as a prohormone reservoir 1, 2. The majority of thyroid hormone effects—including augmentation of cellular respiration, thermogenesis, and metabolism of proteins, carbohydrates, and lipids—are produced by T3 after peripheral conversion from T4 2.
Clinical Measurement Utility
Free T4 measurement is the primary test for thyroid status 4. Serum T3 measurement has limited utility in hypothyroidism because levels often remain normal due to:
- TSH-driven hyperstimulation of remaining thyroid tissue
- Upregulation of type 2 deiodinase enzyme 4
Additionally, T3 levels decrease in severe illness from reduced T4-to-T3 conversion, independent of thyroid disease 4.
Common Pitfalls
When monitoring patients on levothyroxine (T4) replacement, blood should be drawn before dosing because T4 levels transiently increase up to 20% after administration, peaking at 2.5-3.5 hours 4.
In pregnancy, total T4 measurement is preferred over free T4 because direct immunoassays may yield falsely low values, with many patients showing below-normal results in the third trimester even with equilibrium dialysis methods 4.