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)