Free thyroxine is the fraction of the main thyroid hormone circulating unbound to carrier proteins, and it is the fraction available to tissues. Together with TSH it describes how much hormone the thyroid gland is actually delivering. It is the standard measure of thyroid output when pituitary signalling is in question.
No readings for this marker yet.
Raised free T4 most often accompanies an overactive thyroid, whether from Graves' disease, an autonomously functioning nodule, an inflamed gland releasing stored hormone, or an excessive dose of thyroid hormone replacement. Lowered values most often point to an underactive gland, commonly autoimmune thyroiditis or the aftermath of thyroid surgery or radioiodine, and occasionally to reduced pituitary stimulation. Serious non-thyroidal illness can lower it without thyroid disease being present. A single out-of-range result is a reason to repeat the test and discuss it with a doctor, not a conclusion.
Blood is drawn before the daily dose of levothyroxine, since a dose taken hours earlier transiently raises the measured level; biotin supplements interfere with many immunoassays and are usually paused beforehand. Pregnancy, oestrogen and acute illness shift thyroid results, and assays differ enough that trends are best followed in one laboratory. It is read first against TSH, with free T3 and TPO or thyroglobulin antibodies added to clarify the pattern and its cause.
Reference ranges are general adult values and depend on lab, assay, age and sex. For orientation, not diagnosis — discuss results with your doctor.