Free T4 — Free Thyroxine (FT4)

What is Free T4?

Thyroxine (T4) is the principal hormone produced by the thyroid gland, a butterfly-shaped gland in your neck. Most T4 in the blood is bound to carrier proteins, rendering it inactive. Free T4 (FT4) is the small fraction that remains unbound and biologically active — meaning it is available to enter cells and regulate metabolism throughout the body.

The thyroid gland produces T4 in response to thyroid-stimulating hormone (TSH) released by the pituitary gland. Once in tissues, much of the T4 is converted to the more potent hormone T3 (triiodothyronine). Together, T4 and TSH form a tightly regulated feedback loop: when thyroid hormone levels fall, the pituitary releases more TSH to stimulate the thyroid; when levels are high, TSH is suppressed.

Measuring Free T4 alongside TSH gives a much clearer picture of thyroid function than either test alone. It is essential for diagnosing hypothyroidism (underactive thyroid) and hyperthyroidism (overactive thyroid), monitoring thyroid replacement therapy, and investigating symptoms such as fatigue, weight changes, palpitations, and mood disturbance.

Normal Free T4 range

The standard adult reference range for Free T4 is:

• Normal: 9.0–25.0 pmol/L
• Results are always interpreted alongside TSH for a complete thyroid assessment
• In pregnancy, reference ranges differ — specialist assessment is required

What a high Free T4 may indicate

A Free T4 above the reference range, especially when accompanied by a suppressed TSH, typically suggests the thyroid is overactive. Possible causes include:

• Graves' disease — the most common cause of hyperthyroidism; an autoimmune condition stimulating excess thyroid hormone production
• Toxic multinodular goitre — thyroid nodules producing hormone autonomously, independent of TSH
• Thyroiditis — inflammation causing a temporary release of stored hormone (e.g. post-partum thyroiditis, De Quervain's)
• Over-treatment with levothyroxine — too high a dose of thyroid replacement medication
• Excessive iodine intake — supplements or amiodarone (a cardiac drug) can temporarily drive excess hormone production

What a low Free T4 may indicate

A low Free T4, particularly with a raised TSH, points to an underactive thyroid (hypothyroidism). Common causes include:

• Hashimoto's thyroiditis — autoimmune destruction of the thyroid gland; the most common cause in the UK
• Post-thyroid surgery or radioiodine treatment — reduced thyroid tissue producing less hormone
• Iodine deficiency — uncommon in the UK but relevant where dietary intake is very low
• Pituitary or hypothalamic dysfunction — secondary or tertiary hypothyroidism where TSH signalling is impaired (Free T4 low but TSH may be low or normal)
• Certain medications — lithium, amiodarone, and some immunotherapy drugs can impair thyroid function

Get your Free T4 tested at The Vesey

Free T4 is included in the following panels at The Vesey Private Hospital, Sutton Coldfield:

• Thyroid Panel — £148 (TSH + Free T4 + Free T3; full thyroid picture)
• Lifestyle Screen — comprehensive panel including thyroid function
• Advanced Health Panels — full hormonal and metabolic assessment

No fasting required for thyroid tests. Results typically available within 24–48 hours, reviewed by a clinician before release.

Frequently asked questions

Further reading: Free T4 — Lab Tests Online UK · Independent patient information from the British Society for Clinical Biochemistry.