FT4 blood test

Thyroxine is a hormone made by the thyroid gland. The free portion is available to tissues, so FT4 helps show how much thyroid hormone is circulating. A result outside the laboratory range does not provide a diagnosis on its own. A high FT4 with a low TSH can support an overactive thyroid, while a low FT4 with a raised TSH can support an underactive thyroid; patterns can change with illness, pregnancy, supplements and thyroid medicines. Your report should be interpreted using the range printed by that laboratory.

Follow the laboratory instructions and list every medicine and supplement, including biotin, because some products can interfere with thyroid measurements. Do not stop prescribed treatment unless your clinician tells you to. If a result does not fit your symptoms, your clinician may repeat the test or request related thyroid tests. Symptoms such as fatigue can also occur with conditions including Anaemia, Coeliac Disease or Chronic Kidney Disease.

Contact your clinician promptly if an abnormal result is accompanied by marked weakness, confusion, fainting, chest discomfort or a very fast or irregular heartbeat. Seek urgent care for severe breathlessness, collapse or rapidly worsening symptoms. An isolated result should not be used to change medication without advice.

Ask what your FT4 and TSH pattern means together, whether supplements may affect it, when to repeat testing, and which symptoms should prompt earlier review.