What each measures
- Free T4, unbound thyroxine; the storage form circulating freely
- Free T3, unbound triiodothyronine; the active form at receptors
Both have "total" and "free" measurements. Free is what's biologically available, bound forms are reservoir.
Potency difference
T3 is approximately 4× more potent than T4 at thyroid hormone receptors. Most T4 must be converted to T3 to produce biological effect.
Conversion biology
Peripheral tissues convert T4 to T3 using deiodinase enzymes. The conversion can be impaired by:
- Stress / chronic illness (shifts toward inactive reverse T3)
- Selenium deficiency (cofactor)
- Inflammation
- Caloric restriction
- Liver dysfunction
Common patterns
| Pattern | Interpretation |
|---|---|
| TSH high, T4 low, T3 low | Primary hypothyroidism |
| TSH normal, T4 normal, T3 low | Conversion problem (low-T3 syndrome) |
| TSH normal, T4 high-normal, T3 low + rT3 high | Stress / illness pattern |
| TSH low, T4 high, T3 high | Hyperthyroidism |
Optimal ranges
- Free T4: 1.2-1.6 ng/dL (reference often 0.8-1.8)
- Free T3: 3.2-4.2 pg/mL (reference often 2.3-4.2)
- Reference ranges vary by lab; optimal is in upper half
The clinical pearl: Many patients with thyroid-like symptoms have normal TSH and T4 but low Free T3. Standard testing misses this. Comprehensive thyroid panels are required.
Bottom line
Both free T4 and free T3 should be measured for full thyroid evaluation. Conversion problems produce normal T4 with low T3 patterns that TSH alone can miss.
~4x
T3 potency vs T4
Both
required for full evaluation
Conversion
problems missed by TSH+T4 alone