Definition: Free T4 is the unbound, active form of thyroxine, the main hormone the thyroid makes. Most T4 in the blood is attached to proteins and held in reserve. MedlinePlus says the free T4 test is thought to be more accurate than total T4, so it is used more often. It is usually ordered together with TSH.
MedlinePlus explains that free T4 enters body tissues where it is needed, while bound T4 stays in the blood as a backup supply. A total T4 test counts both. Some reports give a "free T4 index" calculated from total T4 instead.
The difference matters with hormones. The Depo-Testosterone label says androgens may lower thyroxine-binding globulin, which lowers total T4, while free thyroid hormone levels stay unchanged and there is no clinical evidence of thyroid dysfunction.
MedlinePlus says a T4 test alone cannot diagnose thyroid problems, so it is read with TSH. The two usually move in opposite directions: low T4 pushes TSH up.
The Genotropin (somatropin) label calls for periodic thyroid function tests, because central hypothyroidism may appear or get worse during growth hormone treatment. MedlinePlus says abnormal T4 levels are sometimes a sign of a pituitary problem that causes too much or too little TSH.
Each lab sets its own reference range, so read your result against the range printed on your own report, and talk with your clinician about what it means for you.
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