Overview: which thyroid tests exist?
| Test | What it measures | When ordered |
|---|---|---|
| TSH (thyroid stimulating hormone) | Pituitary signal to the thyroid — first-line screening test | All thyroid screening; monitoring treatment |
| Free T4 (FT4) | Unbound thyroxine — main thyroid hormone | If TSH abnormal; monitoring levothyroxine |
| Free T3 (FT3) | Unbound triiodothyronine (active form) | If T4 normal but symptoms persist; suspected T3 toxicosis |
| Total T4 / Total T3 | Bound + unbound hormone | Rarely used; affected by protein binding changes |
| Anti-TPO antibodies | Antibodies against thyroid peroxidase enzyme | Suspected Hashimoto thyroiditis (high TSH) |
| Anti-TG antibodies | Antibodies against thyroglobulin | Hashimoto; differentiated thyroid cancer monitoring |
| TSH receptor antibodies (TRAb) | Stimulating or blocking antibodies | Graves disease (low TSH, high T4) |
| Thyroglobulin (Tg) | Protein made by thyroid | Monitoring for thyroid cancer recurrence after surgery/RAI |
| Calcitonin | Hormone from C cells | Medullary thyroid cancer |
Step 1: Always start with TSH
TSH is the single best initial test for thyroid function. It is sensitive to even small changes in thyroid hormone levels. The pituitary acts like a thermostat: if T4 is low, TSH rises to stimulate more production; if T4 is high, TSH falls to suppress the thyroid. A normal TSH (0.4–4.0 mIU/L) makes significant thyroid dysfunction very unlikely and usually no further testing is needed.
Step 2: Interpreting TSH + free T4 patterns
| TSH | Free T4 | Diagnosis |
|---|---|---|
| High TSH | Low T4 | Overt hypothyroidism — treat with levothyroxine |
| High TSH | Normal T4 | Subclinical hypothyroidism — monitor or treat |
| Normal TSH | Normal T4 | Euthyroid — normal thyroid function |
| Low TSH | High T4 | Overt hyperthyroidism — investigate cause |
| Low TSH | Normal T4 | Subclinical hyperthyroidism — monitor or treat |
| Low TSH | Low T4 | Secondary hypothyroidism — pituitary problem; check ACTH |
When to check antibodies
Anti-TPO antibodies — Hashimoto thyroiditis
When to order: if TSH is elevated (hypothyroid or subclinical). A positive anti-TPO confirms autoimmune thyroiditis (Hashimoto) as the cause. High anti-TPO with normal TSH in someone with thyroid symptoms may help predict future hypothyroidism. Anti-TPO levels do not predict disease severity or need for treatment — TSH does.
TRAb (TSH receptor antibodies) — Graves disease
When to order: if TSH is low (hyperthyroid). Positive TRAb confirms Graves disease as the cause of hyperthyroidism. They can also be used to predict remission after antithyroid drug treatment — persistently elevated TRAb after 12–18 months of treatment predicts relapse.
Timing matters: when to test
- Levothyroxine monitoring: test TSH 6–8 weeks after any dose change (T4 has a 7-day half-life, TSH takes weeks to stabilise)
- Take levothyroxine in the morning on an empty stomach, and arrange blood test before that day's dose for accurate monitoring
- Antithyroid drugs (carbimazole/methimazole): test TSH and free T4 every 4–6 weeks until stable
- Pregnancy: TSH targets change each trimester; test at least once per trimester
Questions to ask your doctor
- Should I test free T3 as well as T4?
- What is my anti-TPO level?
- Do I have Graves disease or another cause of hyperthyroidism?
- Am I taking my levothyroxine correctly?
- What TSH level should I aim for?