Free T4 blood test: normal range explained

Free T4 measures thyroxine (T4), the main hormone your thyroid gland releases, in the form that is not bound to carrier proteins. Most T4 in blood is bound to proteins and stays in the bloodstream as a backup supply. The small free share is the part that can enter body tissues and act. MedlinePlus says the free form is considered more accurate, so it is used more often than total T4.

Labs usually print free T4 in ng/dL, and some print pmol/L. Because the result reflects the hormone that is available to your cells, NIDDK says changes in binding proteins do not affect it the way they affect total T4.

Doctors order free T4 to help find and follow an underactive thyroid (hypothyroidism) or an overactive thyroid (hyperthyroidism). It is also used to look into thyroid nodules, a goitre and problems of the pituitary gland, and to check people taking thyroid hormone medicine.

T4 alone is not enough to make a diagnosis, so it is usually ordered with TSH. NIDDK describes TSH as the first check: if TSH is not normal, at least one other test, often free T4, is needed to find the cause.

Several things can move a free T4 result without your thyroid changing, so your doctor reads it with TSH and your history.

• Pregnancy and oestrogen, including birth control pills, change the proteins that carry T4, which affects total T4 more than free T4.
• Biotin (vitamin B7) supplements can interfere with some thyroid assays; your doctor will say whether to pause any medicine or supplement, so do not stop one on your own.
• Severe illness, major surgery and prolonged fasting can lower T4 even when the thyroid is healthy.
• Steroid medicines (corticosteroids) can lower T4, and too much iodine from medicines or seaweed can raise it.
• Thyroid hormone tablets taken shortly before the blood draw can raise the reading, so Testing.com says to have blood drawn before that day's tablet.
• Liver or kidney problems, and inherited changes in the protein that binds T4, can also affect the result.