Reading your thyroid report: what TSH, T3 and T4 actually mean

A thyroid function test usually reports three things: TSH, free T4 and sometimes free T3. Patients are often told only whether the result is "normal" or "abnormal", which is a shame, because the relationship between the three is what actually carries the information.

TSH is a message, not a thyroid hormone

TSH — thyroid stimulating hormone — is made by the pituitary gland, not the thyroid. It is the instruction the pituitary sends when it senses there is not enough thyroid hormone in circulation. So the logic runs backwards from what most people expect:

• High TSH means the pituitary is shouting for more hormone — the thyroid is underactive.
• Low TSH means the pituitary has gone quiet because there is already plenty — the thyroid is overactive.

TSH is also exquisitely sensitive: it moves before T4 does, which is why it changes first and recovers last.

T4 and T3 are the hormones themselves

T4 is the main hormone the thyroid releases; the body converts it into the more active T3 in tissues as required. Free T4 is the portion not bound to protein, and therefore the part that is actually available for use.

The common patterns

High TSH, low free T4 — overt hypothyroidism. Replacement is usually indicated.

High TSH, normal free T4 — subclinical hypothyroidism. This is the one that generates the most unnecessary treatment. Whether it needs anything depends on how high the TSH is, on thyroid antibodies, on your age, on symptoms, and critically on whether you are pregnant or planning to be. Often the correct next step is simply to repeat the test in six to eight weeks.

Low TSH, high free T4 — hyperthyroidism. The cause must then be established, because Graves' disease, a toxic nodule and thyroiditis look similar on this panel but need completely different treatment.

Low TSH, normal free T4 — subclinical hyperthyroidism, which matters more in older people and in anyone with atrial fibrillation or osteoporosis.

Three practical points

Timing of the tablet. If you take levothyroxine, take it after the blood is drawn, not before. Taking it first raises the T4 reading and can lead to a dose being reduced when it should not be.

Illness distorts everything. Thyroid tests done during a significant acute illness, or shortly after, are frequently misleading. Where possible they should be repeated once you are well.

Wait six weeks after a dose change. Rechecking earlier than that shows a level that has not yet settled, and prompts another change that was not needed.

Antibodies

Anti-TPO antibodies indicate Hashimoto's thyroiditis, the commonest cause of hypothyroidism. They help predict whether a borderline result is likely to progress. They do not need repeating once they are known — a falling antibody titre does not mean the thyroid has recovered.

If a report is worrying you, bring it — along with any older ones — and it can be put in context properly. The trend over years is usually more informative than today's figure.