Free T4 (thyroxine)

Ranges vary more between laboratories than for most tests (for example 12 to 22, 9 to 19 or 10 to 20 pmol/L) and differ in pregnancy, so the lab's own printed range matters.

Thyroxine (T4) is the main hormone released by the thyroid gland. Most of it travels in the blood attached to proteins; the free T4 test measures the small unbound fraction that is actually available to the body's tissues. It sets the pace of the body's metabolism.

It is usually requested alongside TSH to see how much thyroid hormone is in circulation. Doctors use it when TSH is outside range, when checking thyroid hormone replacement, and when a pituitary problem is being looked into.

• An overactive thyroid, including Graves disease and thyroid nodules
• Taking more thyroid hormone replacement than the body needs
• Taking the levothyroxine tablet shortly before the blood test
• Some medicines, including amiodarone and heparin given in hospital
• Acute illness, briefly, in the general population

• An underactive thyroid
• Not enough thyroid hormone replacement being taken
• Recovery from a severe illness
• Some medicines, including anti-epileptics and lithium
• Pituitary gland problems, which are uncommon
• Later pregnancy, where the range is lower

• Taking the levothyroxine tablet within a few hours of the test pushes the value up
• Biotin supplements can interfere with the assay on some analysers
• Laboratories use different assays with different ranges
• Some antibodies in the blood can give a falsely high reading on certain analysers

No fasting needed. In someone taking levothyroxine the sample is best taken before that day's tablet. The trend over time, read with TSH, is more useful than one value alone.

Written by a UK consultant physician. Typical UK adult reference ranges; your laboratory's own printed range takes precedence.