# Thyroid Tests Decoded: What TSH, T3, T4 and Antibodies Actually Mean

URL: https://thethyroidologist.com/blog/thyroid-tests-explained/
Published: 2025-11-05
Author: The Thyroidologist Editorial Team
Medical reviewer: Thyroid Specialist

> Thyroid blood tests can be confusing. This guide explains what each test measures, how they work together, and what your results might mean.

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.

## References & further reading

- [Thyroid function tests — NHS](https://www.nhs.uk/conditions/thyroid-function-tests/)
- [Laboratory tests for thyroid disease — American Thyroid Association](https://www.thyroid.org/thyroid-function-tests/)

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