Free Thyroxine

Free T4 (free thyroxine, sometimes written "T4 free" or "serum free thyroxine") is the unbound portion of thyroxine circulating in your blood. T4 is the main hormone the thyroid gland secretes, and the free fraction is the part available to tissues rather than locked to carrier proteins. It works as a storage and reservoir form of thyroid hormone.

A typical reference range is about 0.8–1.8 ng/dL for adults, with slight variation by lab and assay. Free T4 is usually interpreted together with TSH: a normal TSH plus a normal free T4 is the reassuring pattern, while abnormalities in one or both help pinpoint where a thyroid problem sits.

T4 is essentially a prohormone — it's relatively inactive until the body converts it to T3, the biologically active thyroid hormone, by removing an iodine atom (deiodination) in the liver, kidneys, and other tissues. So free T4 is the reservoir and free T3 is the working hormone. Some people carry good free T4 but convert to T3 poorly, which is why measuring both can explain hypothyroid symptoms that free T4 alone would miss.

Free T4 pairs with TSH and free T3 to give a complete thyroid picture, and untreated thyroid dysfunction mimics low testosterone symptoms — fatigue, low libido, weight gain — and can raise SHBG, dragging down your free T. If you don't feel right on a well-dosed TRT protocol, a full thyroid panel including free T4 is a reasonable thing to run with your physician.