Thyroid blood tests measure hormones and antibodies — but the results only make sense together. This guide explains what each test tells you.
TSH — the screening test
TSH (thyroid-stimulating hormone) is produced by the pituitary gland to tell the thyroid how much hormone to make. It is the most sensitive single test for thyroid dysfunction.
- High TSH → thyroid is underactive (hypothyroidism)
- Low TSH → thyroid is overactive (hyperthyroidism) or over-replaced on medication
- Normal TSH → thyroid function is usually adequate
TSH often changes before thyroid hormone levels do — which is why it is the first-line screening test.
Free T4 — confirming hormone levels
Free T4 (FT4) measures the active, unbound thyroxine — the main hormone the thyroid produces. It confirms whether dysfunction is overt or subclinical:
| TSH | Free T4 | Interpretation |
|---|---|---|
| High | Low | Overt hypothyroidism |
| High | Normal | Subclinical hypothyroidism |
| Low | High | Overt hyperthyroidism |
| Low | Normal | Subclinical hyperthyroidism |
Free T3 — the active hormone
Free T3 (FT3) measures triiodothyronine — the most biologically active thyroid hormone. It is less commonly needed but adds information in specific situations, particularly detecting “T3 toxicosis” where T3 is high but T4 is normal.
Antibody tests — identifying the cause
- Anti-TPO — marks autoimmune thyroiditis (Hashimoto’s), the most common cause of hypothyroidism
- Anti-thyroglobulin (anti-Tg) — another autoimmune marker; also used in thyroid cancer surveillance
- TRAb — the specific antibody causing Graves’ disease
Antibody tests do not measure function — they explain why dysfunction occurred.
When more testing is needed
Depending on results, your doctor may add:
- Thyroid ultrasound — evaluates nodules, goitre, and structure
- FNAC biopsy — samples suspicious nodules for cancer evaluation
- Radioactive iodine uptake — distinguishes causes of hyperthyroidism
The bottom line
Thyroid testing is most useful when interpreted as a panel — TSH plus free hormones, with antibodies and imaging added as needed. One abnormal result rarely tells the whole story.