TSH Interpreter (Thyroid Function) — Free Online Calculator
Input
Thyroid Status
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About This Calculator
The TSH Interpreter evaluates thyroid function based on TSH (thyroid-stimulating hormone) and optional Free T4 and Free T3 levels. TSH is the most sensitive initial test for thyroid dysfunction. The pattern of TSH and free hormone levels helps distinguish between overt and subclinical thyroid disease, and guides further workup and treatment decisions.
Formula
Interpretation
| TSH | Free T4 | Diagnosis |
|---|---|---|
| Low | High | Hyperthyroidism |
| High | Low | Hypothyroidism |
| Low | Normal | Subclinical hyperthyroidism |
| High | Normal | Subclinical hypothyroidism |
| Normal | — | Euthyroid |
References
Frequently Asked Questions
Why is TSH the best initial thyroid test?
TSH is the most sensitive indicator of thyroid dysfunction because of the log-linear relationship between TSH and free T4. Small changes in thyroid hormone levels cause large, amplified changes in TSH. TSH abnormalities appear earlier than free T4 changes in developing thyroid disease.
When is Free T3 useful in thyroid evaluation?
Free T3 is primarily useful when hyperthyroidism is suspected but Free T4 is normal (T3 thyrotoxicosis), and in monitoring patients on antithyroid medications. It is generally not needed for hypothyroidism evaluation or routine thyroid screening.
Can medications affect TSH levels?
Yes. Biotin supplements can cause falsely low TSH on immunoassays. Steroids, dopamine, and dobutamine can suppress TSH. Lithium, amiodarone, and iodine-containing contrast can cause either hypo- or hyperthyroidism. Levothyroxine absorption is affected by calcium, iron, and PPIs.
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⚠ Medical Disclaimer
This tool is for educational and informational purposes only. It is not intended to replace professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare professional for clinical decisions.