Thyroid testing: TSH, T3 and T4

TSH is not a thyroid measurement

This catches almost everybody. TSH is made by the pituitary gland, not the thyroid. It is the instruction being sent, not the output being produced.

The pituitary watches circulating thyroid hormone and adjusts. If hormone levels fall, it raises TSH to push harder. If they rise, it backs off. So the relationship runs backwards from what people expect: a high TSH suggests an underactive thyroid, and a low TSH suggests an overactive one.

It is tested first because it is the most sensitive early signal. The pituitary notices a drift before the hormone levels themselves leave the reference range.

The three that get measured

• TSH — the pituitary's instruction. First line, and often sufficient on its own.
• Free T4 — thyroxine not bound to carrier proteins. The thyroid's main output, and mostly a reservoir: it is converted to T3 in the tissues.
• Free T3 — the active hormone, converted from T4 mostly outside the thyroid. Levels in blood are a poor guide to what is happening inside cells, which is why it is not a routine first test.

Free matters. Total T4 and total T3 include hormone bound to carrier proteins and unavailable to tissues, and carrier protein levels shift with pregnancy, oral oestrogen and illness. The free measurements avoid most of that.

Subclinical hypothyroidism, and the argument about it

A raised TSH with a normal free T4 is called subclinical hypothyroidism. The pituitary is pushing harder than usual and the thyroid is still keeping up.

Whether to treat it is genuinely contested. Trials in older adults have been largely disappointing for symptoms and quality of life, while the case is stronger in younger people, in pregnancy, and where thyroid antibodies are present. Guidelines differ, and a clinician who presents this as settled is overstating it in one direction or the other.

Thyroid peroxidase antibodies are the usual next test where it matters, because they indicate autoimmune thyroid disease and change how likely the picture is to progress.

Why the reference range is argued over

Most laboratories report a TSH reference range with an upper limit somewhere between about 4 and 5 mIU/L. That range is derived from a reference population — and if some of that population had undiagnosed thyroid disease, the range is wider than it should be. Proposals to narrow the upper limit have been made repeatedly and have not been generally adopted.

What this means practically: a TSH in the upper part of the range is not automatically fine, and not automatically a problem. It is a number to read against symptoms, antibodies and a repeat.

Things that move TSH without thyroid disease

• Time of day. TSH is higher in the early morning and falls through the day. Draw at a consistent hour.
• Acute illness. Thyroid tests during or just after a significant illness are unreliable. Wait.
• Biotin supplements. High‑dose biotin interferes with many immunoassays and can produce strikingly wrong thyroid results. Stop it for several days before testing and tell the laboratory you were taking it.
• Several medicines, including amiodarone, lithium, and some used in cancer treatment.

Why TSH is in our baseline

It is the most expensive test in our seven‑marker panel at ₡19,000 — nearly a fifth of the total — and it was cut from an earlier draft to bring the price down, then put back.

The reason it went back: the two complaints people arrive with are fatigue and weight that will not move, and an underactive thyroid is the commonest reversible cause of both. Missing it and prescribing a metabolic medicine instead would be treating the wrong thing at considerable expense.

The boundary

A thyroid result is read against your symptoms, your medicines, your age and a repeat weeks later — not on its own. What yours means, and whether it needs anything done about it, is a clinical judgement rather than a threshold.