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The Triiodothyronine (T3) test measures the total level of T3 — the most biologically potent thyroid hormone — in the blood. T3...
Also known as: Total T3 Test · Serum T3 Test · T3 Level Test · Triiodothyronine Level Test
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*Inclusive of home collection charges
The Triiodothyronine (T3) test measures the total level of T3 — the most biologically potent thyroid hormone — in the blood. T3 is three to four times more potent than T4 at the cellular level and is responsible for the majority of thyroid hormone's metabolic effects. While most circulating T3 is produced by peripheral conversion of T4, the thyroid also secretes T3 directly — particularly in hyperthyroidism. T3 measurement is most clinically valuable in diagnosing hyperthyroidism and identifying T3 toxicosis — where T3 is elevated while T4 remains normal. No fasting required.
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What Does the Triiodothyronine (T3) Test Measure?
| Level / Result | Possible Interpretation |
|---|---|
| 80–200 ng/dL (Normal) | Normal T3 levels — adequate thyroid hormone activity |
| Elevated T3 + Suppressed TSH + Normal T4 (T3 toxicosis) | T3-predominant hyperthyroidism — Graves' disease, toxic nodule; T3 is the critical diagnostic parameter |
| Elevated T3 + Elevated T4 + Suppressed TSH | Overt hyperthyroidism — T3 elevation confirms severity; both hormones elevated |
| Normal T3 + Elevated TSH (Hypothyroidism) | T3 preserved in hypothyroidism — not useful for diagnosing underactive thyroid; use FT4 and TSH |
| Low T3 + Normal T4 + Normal TSH (Low T3 syndrome) | Non-thyroidal illness / euthyroid sick syndrome — T3 conversion impaired in acute illness; not true hypothyroidism |
| Elevated T3 + Normal TSH + Normal T4 | Possible T3 supplementation, TBG elevation (pregnancy/oestrogens), or laboratory variation |
Important Note: T3 is the least useful thyroid test for diagnosing hypothyroidism — it is preserved until very late in the disease course. T3 is most valuable in hyperthyroidism diagnosis. Total T3, like total T4, is affected by protein levels — Free T3 is preferred when protein abnormalities are present. T3 falls markedly in non-thyroidal illness (sick euthyroid syndrome) without reflecting true thyroid disease. All results must be reviewed by a physician.
T3 Toxicosis — The Hyperthyroidism That T4 Misses: In some patients with early or mild Graves' disease and in many cases of toxic thyroid nodules, the overactive thyroid tissue preferentially secretes T3. At this stage, T4 may still be within the normal range while T3 is markedly elevated and TSH is suppressed. Ordering only T4 would miss this diagnosis. T3 toxicosis is more common in iodine-deficient populations and in patients with long-standing multinodular goitre — precisely the populations common in India. The full Thyroid Profile (T3 + T4 + TSH) therefore provides more complete diagnostic coverage than any single thyroid hormone test.
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