What Is Free T4 in a Blood Test?
Free T4 measures the unbound, biologically active form of thyroxine — the main thyroid hormone. Preferred over total T4 because it is not affected by protein-binding abnormalities.
Normal Range
- 0.8–1.8 ng/dL
How Free T4 Is Used with TSH
- High TSH + Low Free T4 = primary hypothyroidism (overt)
- High TSH + Normal Free T4 = subclinical hypothyroidism
- Low TSH + High Free T4 = primary hyperthyroidism
- Low TSH + Normal Free T4 = subclinical hyperthyroidism
- Low TSH + Low Free T4 = central hypothyroidism (pituitary problem)
- Normal TSH + Normal Free T4 = normal thyroid function
When Is Free T4 Ordered?
- Abnormal TSH result
- Monitoring thyroid hormone replacement
- Suspected pituitary disease
- Pregnancy (TSH ranges change)
Source: NLM MedlinePlus, American Association for Clinical Chemistry.
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