How thyroid function test interpretation works
Thyroid status is read from the relationship between TSH (pituitary signal) and Free T4/Free T3 (the thyroid's own output) — not from any single value alone. TSH is inversely and logarithmically sensitive to thyroid hormone, so it's the first-line screening test; Free T4 (and sometimes Free T3) confirms and characterises the pattern.
Normal reference ranges
| Test | Typical adult reference range |
|---|---|
| TSH | 0.4 – 4.0 mIU/L |
| Free T4 (FT4) | 0.8 – 1.8 ng/dL |
| Free T3 (FT3) | 2.3 – 4.2 pg/mL |
Reference ranges vary meaningfully between labs and assay platforms — always interpret against the range printed on the actual report, not these general figures. Pregnancy, critical illness, and certain medications (amiodarone, lithium, biotin supplements) also shift these ranges or interfere with the assay.
Clinical use
- TSH is the best single screening test in an otherwise well outpatient with normal pituitary function, precisely because it responds first and most sensitively to small changes in thyroid hormone.
- Subclinical patterns need context, not automatic treatment: subclinical hypothyroidism and subclinical hyperthyroidism are often confirmed on a repeat test 6–12 weeks later before deciding whether to treat, especially when TSH is only mildly abnormal.
- Discordant patterns (TSH and FT4 both high, or both low) are red flags for pituitary disease, thyroid hormone resistance, or an assay interference (e.g. biotin) — these warrant repeat testing and endocrinology input rather than a routine primary-thyroid diagnosis.
- Sick euthyroid syndrome: acute severe illness can transiently distort TFTs (typically low FT3, variable TSH/FT4) without true thyroid disease — repeat testing after recovery is more reliable than interpreting TFTs drawn during critical illness.
Frequently asked questions
What is a thyroid function test?
A thyroid function test (TFT) is a blood test panel — usually TSH and Free T4, sometimes with Free T3 — used to check whether the thyroid gland is underactive, overactive, or working normally.
What does a high TSH mean?
A high TSH means the pituitary is working harder to stimulate the thyroid — usually because the thyroid itself isn't producing enough hormone. If Free T4 is also low, that's primary hypothyroidism; if Free T4 is still normal, that's subclinical hypothyroidism.
What does a low TSH mean?
A low TSH means the pituitary has dialled down its signal — usually because the thyroid is already producing enough or too much hormone. If Free T4 is high, that's primary hyperthyroidism; if Free T4 is still normal, that's subclinical hyperthyroidism.
What is subclinical hypothyroidism?
A mildly elevated TSH with a Free T4 that's still within the normal range. It doesn't always need treatment — the decision depends on how high the TSH is, symptoms, antibody status, and patient factors like pregnancy or cardiovascular risk.
Why check Free T3 as well as TSH and Free T4?
A small number of hyperthyroid patients have a normal Free T4 but an elevated Free T3 ("T3 toxicosis") — this pattern is only caught if FT3 is checked, so it's added when hyperthyroidism is clinically suspected despite a normal-looking TSH/FT4 pair.
Can thyroid function tests be wrong or misleading?
Yes — acute illness, pregnancy, certain medications (amiodarone, lithium, steroids), and biotin supplements (which can interfere with many immunoassays) can all distort results without true thyroid disease. Isolated or discordant abnormal results are usually repeated before acting on them.
How often should thyroid function be rechecked after starting treatment?
Typically 6–8 weeks after starting or changing a dose of thyroid medication, since TSH takes that long to fully reflect the new dose — checking earlier can be misleading.
References
- Jonklaas J, et al. Guidelines for the Treatment of Hypothyroidism (American Thyroid Association). Thyroid. 2014;24:1670–1751.
- Ross DS, et al. 2016 American Thyroid Association Guidelines for Diagnosis and Management of Hyperthyroidism and Other Causes of Thyrotoxicosis. Thyroid. 2016;26:1343–1421.
- Garber JR, et al. Clinical Practice Guidelines for Hypothyroidism in Adults (AACE/ATA). Endocr Pract. 2012;18:988–1028.