- Biomarker 01
- TSH
- Biomarker 02
- Free T4
Quick answer.
TSH is the screening test; Free T4 measures actual hormone. TSH reflects the brain's demand for thyroid hormone - high TSH means the body wants more (hypothyroid). Free T4 shows supply. Optimal TSH: 1.0-2.0 mIU/L. Use both to distinguish primary thyroid disease from pituitary issues.
Key differences.
- What it measures — TSH: Pituitary signal requesting thyroid hormone; FT4: Active thyroid hormone available to tissues.
- Source — TSH: Pituitary gland; FT4: Thyroid gland.
- Response time — TSH: Changes over weeks; FT4: Changes over days.
- Screening utility — TSH: Primary screening test; FT4: Confirmatory/follow-up test.
When to use each.
TSH
- Initial thyroid screening
- Monitoring thyroid medication
- Detecting subclinical thyroid disease
- Annual health screening
Free T4
- Confirming abnormal TSH
- Distinguishing hyperthyroidism types
- Monitoring thyroid storm risk
- Pituitary disease suspected
Use both
- Complete thyroid evaluation
- Unusual symptoms with normal TSH
- Central hypothyroidism suspected
- Thyroid medication optimization
Side-by-side.
| Feature | TSH | FT4 |
|---|---|---|
| What it tells you | Brain's thyroid hormone demand | Actual thyroid hormone supply |
| Optimal range | 1.0-2.0 mIU/L | 1.0-1.5 ng/dL |
| Lab normal range | 0.4-4.0 mIU/L | 0.8-1.8 ng/dL |
| High value means | Hypothyroid (underactive) | Hyperthyroid (overactive) |
| Low value means | Hyperthyroid or pituitary issue | Hypothyroid or low production |
| Cost | ~$20-40 | ~$30-50 |
FAQs.
Why check TSH first instead of thyroid hormones directly?
TSH is the most sensitive early indicator. The pituitary constantly monitors thyroid levels and adjusts TSH before T4 levels change significantly. A subtle thyroid problem shows up in TSH weeks before T4 becomes abnormal.
Can TSH be normal with thyroid disease?
Yes, in central (pituitary) hypothyroidism, the pituitary fails to produce TSH despite low T4. This is rare but important. If symptoms suggest hypothyroidism but TSH is normal, checking Free T4 can reveal the problem.
What does high TSH with normal T4 mean?
This is subclinical hypothyroidism - the thyroid is struggling but still producing enough hormone. The pituitary is working harder (high TSH) to maintain normal T4. Many experts treat if TSH exceeds 10, or lower with symptoms.
Should I also check T3?
Free T3 is useful in specific situations: suspected T4-to-T3 conversion problems, hyperthyroidism evaluation, or symptoms despite normal TSH/T4. For routine screening, TSH +/- Free T4 is sufficient.
Verdict.
TSH is the first-line screening test - if it's normal (ideally 1.0-2.0 for longevity), thyroid function is likely adequate. Add Free T4 when TSH is abnormal or symptoms persist despite normal TSH. For comprehensive thyroid evaluation, check both. The inverse relationship is key: high TSH + low T4 = hypothyroid; low TSH + high T4 = hyperthyroid.