In tft which should we check free t3,t4 or total t3,t4
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thyroid function testing free T4 total T4 T3 guidelines TSH
For routine thyroid function testing, check:
- TSH + free T4 (FT4): the usual first-line tests.
- T3: add it mainly if TSH is low and hyperthyroidism is suspected, especially when FT4 is normal. In this setting, total T3 is generally more useful and reliable than free T3.
So, usually:
| Situation | Preferred test |
|---|---|
| Initial TFT evaluation | TSH, then FT4 if needed |
| Suspected hypothyroidism | TSH + FT4 |
| Suspected hyperthyroidism | TSH + FT4 + total T3 |
| Pregnancy, estrogen/OCP use, altered binding proteins | TSH + FT4, with pregnancy-specific interpretation |
| Free T3 | Usually not routinely needed or preferred |
Why not total T4 routinely? Total T4 and total T3 are influenced by thyroid-binding proteins. For example, pregnancy and estrogen-containing oral contraceptives can raise total T4/T3 without true hyperthyroidism. Free T4 better represents biologically available hormone. Over 99% of circulating T4 is protein-bound, while free T4 produces the biological effect (Henry's Clinical Diagnosis and Management by Laboratory Methods).
The American Thyroid Association guidance likewise states that FT4 with TSH more accurately reflects thyroid function, and that free T3 is often less reliable and not typically helpful.
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