Free T4 (FT4) test explained

Thyroxine (T4) is the main hormone the thyroid produces, and it regulates metabolism, heart rhythm, body temperature and how the body uses energy. Most of the T4 in the blood is bound to carrier proteins and is biologically inactive; only a small part — free T4 (FT4) — circulates unbound and can enter cells and act on tissues. That is why FT4 is considered a more accurate marker of thyroid function than total T4: it is not affected by changes in the amount of carrier proteins (for example during pregnancy or while taking hormonal medicines).

The FT4 test is most often done together with TSH (thyrotropin) — the hormone the pituitary gland makes to stimulate the thyroid. When the thyroid is working too weakly (hypothyroidism), TSH usually rises and FT4 falls; when it is working too strongly (hyperthyroidism), TSH falls and FT4 rises. Assessed together, the two results make it possible to tell a thyroid disorder from a disorder of the pituitary gland or the hypothalamus. For the wider picture, see thyroid tests.

• When a TSH result is abnormal — to clarify whether the thyroid is working too weakly or too strongly
• With signs of hyperthyroidism: weight loss despite a normal or increased appetite, a fast heartbeat, nervousness, sweating, tremor, sleep problems
• With signs of hypothyroidism: tiredness, weight gain, intolerance of cold, joint or muscle aches, dry skin and hair, constipation
• To monitor an already diagnosed thyroid condition, or how well treatment with thyroid hormones is working
• When a pituitary problem is suspected, or when the causes of infertility are being investigated
• When a thyroid nodule or an enlarged gland (goitre) has been found on palpation or on an imaging test

A raised FT4 is most often linked to a thyroid that is working too actively (hyperthyroidism). The most common causes are Graves' disease, inflammation of the thyroid (thyroiditis) or too high a dose of a thyroid hormone preparation. TSH, assessed alongside it, helps to establish the exact cause.

• Graves' disease — an autoimmune condition that makes the thyroid produce too much hormone
• Inflammation of the thyroid (thyroiditis)
• A toxic goitre or a toxic nodule
• Too much iodine
• A pituitary tumour that stimulates the thyroid
• Too high a dose of a thyroid hormone preparation

Symptoms that can go with it: weight loss even though eating is unchanged or increased, a fast heartbeat, nervousness, sleep problems, tiredness, tremor, heavy sweating, more frequent bowel movements, and a goitre (an enlarged thyroid).

What to discuss with a doctor: the TSH result, which helps to establish whether the problem is in the thyroid or in the pituitary gland; whether too high a dose of thyroid hormone is being taken; thyroid antibodies (anti-TPO, anti-TSHR) if an autoimmune cause is suspected; and an ultrasound scan of the thyroid if a nodule or a goitre can be felt.

A low FT4 most often indicates a thyroid that is working too weakly (hypothyroidism), for example because of Hashimoto's disease, or a severe acute illness. TSH, assessed alongside it, helps to tell a thyroid disorder from a pituitary problem.

• Hashimoto's disease and other thyroid disorders that cause hypothyroidism
• A severe acute illness
• Pituitary disorders that result in a lack of TSH
• Iodine deficiency
• Congenital hypothyroidism

Symptoms that can go with it: tiredness, weight gain, intolerance of cold, joint and muscle aches, dry skin and hair, irregular periods, fertility problems, a slow heart rate, low mood and constipation.

What to discuss with a doctor: the TSH result, which helps to separate thyroid causes from pituitary ones; thyroid antibodies (anti-TPO) if Hashimoto's disease is suspected; whether any medicines being taken could affect the result; and when to repeat the test if treatment is being started or adjusted.

At the Medicina practica laboratory, a free thyroxine (FT4) test costs from €13 (September 2026 pricing). A family doctor can order the test free of charge when there are indications for it.