Free T4
The active, unbound form of thyroxine, a main thyroid hormone. Read with TSH to assess how the thyroid is working.
Typical reference range
| Range | Unit |
|---|---|
| 11–22 | pmol/l |
What Free T4 measures
Free T4 (free thyroxine) is the small, unbound fraction of thyroxine, the main hormone made by your thyroid gland. Most thyroxine in the blood travels attached to carrier proteins, but only the free portion is biologically active and available to your tissues, so measuring the free fraction gives a clearer picture than total T4, which shifts when protein levels change. Thyroxine production is regulated by TSH, a signal released by the pituitary gland in the brain. Because of this feedback loop, Free T4 is rarely read alone. A clinician usually interprets it alongside TSH, and sometimes Free T3 and thyroid antibodies such as TPO, to understand how the thyroid and pituitary are working together.
What a high Free T4 can mean
A high Free T4 can suggest an overactive thyroid (hyperthyroidism), where the gland is producing more hormone than the body needs. Conditions commonly associated with this pattern include Graves disease, thyroiditis (inflammation of the gland), overactive nodules, and taking more thyroid medication than the body requires.
Because thyroid hormone helps set the pace of metabolism, a raised level is often linked with sped-up symptoms, which can include:
- a fast or pounding heartbeat
- feeling anxious, restless, or shaky
- unexplained weight loss despite a normal or increased appetite
- heat intolerance and increased sweating
- trouble sleeping
Free T4 is interpreted together with TSH. A high Free T4 paired with a low TSH is the classic pattern of primary hyperthyroidism. These results describe a pattern, not a diagnosis, and a clinician interprets them in the context of your symptoms, history, and any further testing.
What a low Free T4 can mean
A low Free T4 can suggest an underactive thyroid (hypothyroidism), where the gland is not producing enough hormone. A common cause is Hashimoto disease, an autoimmune condition in which the immune system gradually reduces thyroid function. Less often, a low result points to a pituitary or hypothalamic cause (central hypothyroidism), where the brain does not send enough TSH signal, so TSH may be low or normal rather than high.
Because thyroid hormone sets the pace of metabolism, a low level is often linked with slowed-down symptoms, which can include:
- persistent tiredness or low energy
- weight gain or difficulty losing weight
- feeling cold when others are comfortable
- dry skin, constipation, or low mood
- slower thinking or sluggishness
The TSH pairing matters. A low Free T4 with a high TSH typically reflects primary hypothyroidism, while a low Free T4 with a low or normal TSH may point toward a pituitary cause. Discuss any results with a clinician, who interprets them alongside your symptoms and history.
Track your Free T4 over time in Quanome — it reads your lab reports (PDF or photo) on your device, charts each marker against its range, and an AI coach explains what your trends mean alongside your Apple Health and DNA.