Free T4

Free T4 measures the unbound, active form of thyroxine, the main hormone produced by the thyroid gland. It is one of the most important tests for evaluating thyroid function.

Category: Thyroid Panel

Also known as: free t4, ft4, thyroxine free, free thyroxine

Reference range: 0.8–1.8 ng/dL (Source: ABIM)

Standard unit: ng/dL

High values may indicate

High free T4 may indicate hyperthyroidism, Graves' disease, thyroiditis, or excessive thyroid hormone medication.

Low values may indicate

Low free T4 may indicate hypothyroidism, pituitary disorder, severe illness, or iodine deficiency.

Source: MedlinePlus, U.S. National Library of Medicine

Track your Free T4 over time with BloodId