Thyroid Function Test Interpreter
Interpret TSH, free T4, and free T3 together to identify the common thyroid function patterns — overt and subclinical hypothyroidism and hyperthyroidism, and non-thyroidal illness.
Formula & Reference
| Variable | Symbol | Formula | Units |
|---|---|---|---|
| Thyroid Function Test Interpreter | — | Pattern determined by TSH combined with free T4 and free T3 | interpretation |
Step-by-Step Examples
TSH 7.8 mIU/L, free T4 1.1 ng/dL.
- TSH above the 4.0 upper reference limit
- Free T4 within the normal range
- This combination defines the subclinical pattern
- Repeat testing before treatment decisions is standard
TSH 0.02 mIU/L, free T4 3.4 ng/dL, free T3 9.1 pg/mL.
- TSH suppressed well below 0.4
- Free T4 clearly above the upper limit
- Free T3 also markedly elevated
TSH 2.1 mIU/L, free T4 1.3 ng/dL.
- TSH comfortably within 0.4–4.0
- Free T4 within 0.8–1.8
- No abnormal pattern identified
Real-World Applications
Common Mistakes to Avoid
Non-thyroidal illness syndrome produces abnormal results that do not reflect true thyroid dysfunction. Testing is best deferred until recovery unless thyroid disease is strongly suspected.
TSH varies with time of day, illness, and biotin supplementation, which interferes with many assays. Confirmation on a repeat sample is standard before diagnosis.
Reference ranges differ in pregnancy, in older adults where a modestly higher TSH may be normal, and between laboratories using different assays.