Also seen on reports as: Total T4, Free T4, FT4, Tetraiodothyronine
What is T4?
T4 (Thyroxine) is the main hormone the thyroid gland releases. The thyroid produces several times more T4 than T3. T4 then circulates in the blood and is converted to the more active T3 in the liver, kidneys, and other tissues, as the body needs it.
Lab reports usually show one of two forms — Total T4 (all the T4 in the blood, including the portion bound to proteins) or Free T4 (the small fraction that is unbound and biologically available). Free T4 is generally considered the more reliable indicator of thyroid status in most clinical situations.
The test is run on a small blood sample drawn from a vein. NABL-accredited Indian labs typically report Total T4 in µg/dL and Free T4 in ng/dL or pmol/L.
Why doctors order it
T4 is most commonly ordered alongside TSH to confirm a suspected thyroid problem, or to follow up an abnormal TSH. Together, TSH and Free T4 give the best first picture of thyroid function in adults.
Doctors order T4 when symptoms suggest hypothyroidism (tiredness, weight gain, cold intolerance, constipation, dry skin, hair thinning, irregular periods) or hyperthyroidism (palpitations, unexplained weight loss, heat intolerance, tremor, anxiety, frequent bowel movements). It is also part of the standard antenatal and pre-conception thyroid panel.
In someone already on thyroxine medication (Eltroxin, Thyronorm, etc.), T4 is sometimes monitored alongside TSH to check that the dose is in range — particularly during dose changes, pregnancy, or when symptoms persist despite a normal TSH.
What the result means
A normal TSH with a normal Free T4 is the simplest pattern — thyroid function is in range. A high TSH with a low Free T4 is the classic pattern of overt hypothyroidism. A low TSH with a high Free T4 (often with high T3) is the classic pattern of hyperthyroidism.
Subclinical hypothyroidism (mildly elevated TSH with a normal Free T4) is common in Indian adults, particularly women. Whether to treat depends on the TSH value, antibody status, age, pregnancy plans, and symptoms — your doctor weighs the full picture rather than the T4 alone.
Free T4 is preferred over Total T4 in most situations. Pregnancy, oral contraceptives, oestrogen therapy, and some liver conditions raise the binding proteins that carry T4 in the blood, which can lift Total T4 without changing the biologically active free portion.
Reference range
Reference ranges vary between Indian labs — interpret against the range on the same report
- Total T4 — typical adult range
- Roughly 5.0 – 12.0 µg/dL (lab-dependent)
- Free T4 — typical adult range
- Roughly 0.7 – 1.9 ng/dL (lab-dependent)
- Hypothyroidism (overt)
- Free T4 low — usually with high TSH
- Hyperthyroidism
- Free T4 high — usually with low TSH
- Subclinical hypothyroidism
- Free T4 normal with mildly raised TSH
T4 reference ranges differ between NABL-accredited labs because thyroid assays are not fully standardised across manufacturers. Pregnancy uses different (trimester-specific) Free T4 ranges; newborns are screened separately.
Common factors that affect the result
Pregnancy, oral contraceptives, hormone replacement therapy, oestrogen treatment, and chronic liver disease all raise the binding proteins that carry T4 in the blood. This elevates Total T4 without changing what is actually biologically active. Free T4 is much less affected by these factors and is preferred for interpretation in those situations.
Acute illness, hospitalisation, severe burns, and major surgery can transiently shift T4 levels — sometimes lowering Free T4 (part of the 'non-thyroidal illness' or 'sick euthyroid' pattern). This usually resolves as the underlying illness recovers and does not by itself indicate a thyroid disease.
Several medications affect T4 — biotin supplements (commonly taken for hair and nail health in India) can produce false readings on many assays and should be stopped 48–72 hours before testing. Amiodarone, lithium, and some anti-seizure medications can also shift T4 either up or down. Iron, calcium, antacids, and some Indian Ayurvedic supplements can interfere with thyroxine absorption from medication and indirectly affect levels.
When to talk to your doctor
- Your Free T4 is outside the lab's reference range for the first time, particularly if TSH is also abnormal.
- Your Free T4 is low with a high TSH — this combination usually warrants thyroid hormone replacement.
- Your Free T4 is high with a low TSH and you have symptoms of hyperthyroidism — palpitations, weight loss, tremor, heat intolerance.
- You are on thyroid medication and Free T4 has moved significantly above or below your doctor-set target.
- You are planning pregnancy, are pregnant, or are post-partum — Free T4 targets shift in pregnancy and need obstetric input.
Related tests
Sources
- Indian Thyroid Society Guidelines for Management of Hypothyroidism
- ATA Clinical Practice Guidelines for Hypothyroidism in Adults — American Thyroid Association
- WHO Thyroid Disorders Guidance — World Health Organization
- FOGSI Guidelines on Thyroid in Pregnancy — Federation of Obstetric and Gynaecological Societies of India
See our medical disclaimer for what this content is and is not.