Comparison · Biomarker vs Biomarker
TSH vs free T4
TSH comes from the pituitary and free T4 from the thyroid, and they move in opposite directions. Reading them together tells whether a thyroid problem is mild or severe and whether it starts in the gland or in the pituitary.
| Aspect | TSH (thyroid-stimulating hormone) | Free T4 (thyroxine) |
|---|---|---|
| Source | Pituitary gland | Thyroid gland (as thyroxine, T4) |
| Direction in an underactive thyroid | Rises, often to many times the upper limit | Falls |
| Direction in an overactive thyroid | Suppressed towards zero | Rises |
| Sensitivity | Very high: changes exponentially with small changes in T4 | Changes more slowly and stays within range in mild (subclinical) disease |
| Speed after a dose change | Takes about 6 weeks to settle | Changes within days |
| When it misleads | Pituitary disease, the first trimester, severe illness, biotin supplements | Binding-protein changes are avoided by measuring the free fraction; heparin and biotin can interfere |
Where TSH (thyroid-stimulating hormone) fits
Screening for thyroid disease and adjusting levothyroxine, because it is the most sensitive single measure
Where Free T4 (thyroxine) fits
Judging severity, following treatment in the first weeks, and in pituitary disease where TSH cannot be trusted
Educational content. A comparison explains what each test, marker or medicine is for and how they differ. It is educational and does not say which one a person should have; that decision belongs to the clinician who knows the person and the question.