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The Thyroxine (T4) test measures the total level of thyroxine — the primary hormone secreted by the thyroid gland — in the bloo...
Also known as: Total T4 Test · Serum T4 Test · Thyroxine Level Test · Tetraiodothyronine Test
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*Inclusive of home collection charges
The Thyroxine (T4) test measures the total level of thyroxine — the primary hormone secreted by the thyroid gland — in the blood, including both protein-bound (inactive) and free (active) fractions. T4 is the thyroid's main product and serves as a circulating reservoir that peripheral tissues convert to the more potent T3 as needed. Total T4 is a useful and widely available thyroid hormone marker — most meaningful when interpreted alongside TSH, and in patients without conditions that alter carrier proteins. No fasting required.
Key benefits of this package include:
What Does the Thyroxine (T4) Test Measure?
| Level / Result | Possible Interpretation |
|---|---|
| 5.0–12.0 µg/dL (Normal) | Normal thyroid hormone production |
| < 5.0 µg/dL (Low T4) + Elevated TSH | Overt hypothyroidism — levothyroxine therapy indicated |
| < 5.0 µg/dL (Low T4) + Normal/Low TSH | Central hypothyroidism or severe non-thyroidal illness — specialist evaluation required |
| > 12.0 µg/dL (Elevated T4) + Suppressed TSH | Overt hyperthyroidism — antithyroid treatment required |
| Elevated T4 + Normal TSH + Pregnancy | Physiological TBG rise in pregnancy — use Free T4 for accurate assessment |
| Low T4 + Low TSH + Severe illness | Non-thyroidal illness syndrome (euthyroid sick syndrome) — T4 falls in critical illness; interpret with caution |
Important Note: Total T4 must be interpreted alongside TSH. In pregnancy, protein-altering conditions, and critical illness, Free T4 is preferred. T4 values are age-dependent and slightly higher at birth. Total T4 trending over time is useful for monitoring known thyroid disease. All results must be reviewed by a physician.
Non-Thyroidal Illness Syndrome (Euthyroid Sick Syndrome): In critically ill patients — sepsis, major surgery, starvation, burns — total T3 and T4 fall dramatically despite normal thyroid function, while TSH remains paradoxically normal or low. This is caused by reduced peripheral T4-to-T3 conversion and altered protein binding. This 'sick euthyroid' pattern must not be mistaken for true thyroid disease — treatment with thyroid hormones does not benefit these patients. Thyroid function testing is generally not recommended during acute critical illness unless specific thyroid disease is clinically suspected.
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