✍️ Edited by Farhan Ahmed, Editor, MedTestBD · Last updated: August 2026
What is the T4 Test?
The T4 test measures the level of thyroxine — one of the two main hormones secreted by the thyroid gland — in the blood. T4 is produced in larger quantities than T3 but is less biologically active; most of it is converted to the more potent T3 in body tissues. T4 can be measured as total T4 (bound and unbound) or Free T4 (the active unbound fraction). It is routinely ordered alongside TSH to assess overall thyroid function.
Why is the T4 Test Done?
- To confirm hypothyroidism — a low T4 alongside a high TSH confirms an underactive thyroid
- To confirm hyperthyroidism — a high T4 alongside a low TSH confirms an overactive thyroid
- To monitor thyroid hormone replacement therapy in patients on levothyroxine
- To evaluate thyroid function in pregnant women — thyroid disorders are common in pregnancy and affect fetal development
- To investigate symptoms of thyroid disease such as fatigue, weight changes, hair loss, palpitations, or cold intolerance
- To assess pituitary-thyroid axis function alongside TSH and T3
Normal T4 Levels
| Type | Normal Range |
|---|---|
| Total T4 (adults) | 5.0 – 12.0 µg/dL (64 – 154 nmol/L) |
| Free T4 (adults) | 0.8 – 1.8 ng/dL (10 – 23 pmol/L) |
| Pregnancy | Total T4 rises due to increased binding proteins — use trimester-specific ranges |
Free T4 is preferred over total T4 in most clinical situations because it is not affected by changes in thyroid-binding protein levels — which rise in pregnancy, with oral contraceptive use, and in liver disease. Your doctor will almost always interpret T4 results alongside TSH for the most accurate assessment of thyroid function.
How to Prepare for the T4 Test
- No fasting is required
- If you take levothyroxine (thyroid medication), ask your doctor whether to take your morning dose before or after the blood draw — many prefer the sample taken before the dose for consistency
- Stop biotin supplements at least 24–48 hours before the test if instructed — biotin interferes with immunoassay-based thyroid tests
- Inform the lab about any steroids, oestrogen therapy, or amiodarone you are taking, as these affect T4 levels and binding
Normal ranges cited on this page are based on MedlinePlus and WHO guidance. Prices are collected from public sources and may change — confirm with the facility before visiting. This information is for general reference only and is not medical advice. Always consult a qualified physician.
