How we read a thyroid panel: TSH, free T4 and free T3

Thyroid stimulating hormone is on almost every check-up in the UAE, and almost every check-up stops there. A TSH on its own answers one question, whether the pituitary thinks the thyroid is doing enough, and leaves the interesting ones open. This note explains how we read a thyroid set of TSH, free T4 and free T3, what a borderline result usually means, and when it is time to involve a doctor.

How the three fit together

The thyroid makes thyroxine, T4, which is mostly a storage hormone. Tissues convert it to triiodothyronine, T3, which is the active form. The pituitary gland watches the level of both and releases TSH to ask the thyroid for more. So TSH moves in the opposite direction to the thyroid hormones: a struggling thyroid produces a high TSH, an overactive one a low TSH.

• TSH is the most sensitive single marker, because the pituitary responds to small changes in T4 with large changes in TSH.
• Free T4 is the hormone actually available to tissues, unbound from carrier proteins.
• Free T3 is the active hormone, and the one that most closely tracks how someone feels.

Reading all three at once is what turns a screen into an interpretation. A TSH of 5.5 with a free T4 in the middle of the range is a different picture from the same TSH with a free T4 at the bottom of it.

The borderline TSH

Most laboratory reference ranges for TSH run from about 0.4 to 4.0 or 4.5 mIU/L. A result between 4 and 10 with a normal free T4 is called subclinical hypothyroidism, and it is one of the most common findings in a large panel. It is also one of the most over-treated.

Before reading anything into a mildly raised TSH, we look at four things.

Was it a single reading?

TSH varies through the day and with sleep, illness and stress. It is highest late at night and in the early morning and lowest in the afternoon. A recent viral illness can raise it for weeks. Around a third of mildly raised results are normal on repeat, which is why guidelines ask for a second test two to three months later before anyone is labelled.

Is free T4 normal?

If it is, the thyroid is still meeting demand. If free T4 is low as well, the picture is overt hypothyroidism and it needs a clinician regardless of how the person feels.

Are there antibodies?

Thyroid peroxidase antibodies indicate autoimmune thyroiditis, the commonest cause of an underactive thyroid in the UAE and most of the world. A raised TSH with positive antibodies is far more likely to progress than one without, and it changes how often to recheck. Antibodies are not in the routine panel and are worth asking a doctor about if TSH is persistently raised.

Is there anything else going on?

Iron deficiency, low vitamin D, recent pregnancy, biotin supplements and a number of medications all affect thyroid results or the assay itself. Biotin in particular, common in hair and nail supplements, can make TSH read falsely low and free T4 falsely high. Tell the person taking the sample if you take it. Some laboratories and the American Thyroid Association ask for a gap between the last dose and the sample, but do not stop any medicine or supplement unless your doctor tells you to.

Age changes what normal means

TSH rises slowly with age, and an older adult with a TSH of 5 or 6 and no symptoms may simply be normal for their age. Treating that number to bring it into a range built from younger people does not help and can cause harm. In someone under forty the same value deserves more attention, especially if antibodies are present or there is a family history.

The low TSH

A TSH below the range with normal free T4 and T3 is subclinical hyperthyroidism. In the UAE the commonest causes are thyroid nodules, early Graves' disease, and, very often, taking too much thyroid medication. A persistently low TSH matters for bone and heart rhythm even without symptoms, so it is one to take to a doctor rather than to watch.

What we say in the app

A borderline TSH in Clare arrives with the other two markers beside it, the ferritin and vitamin D that often travel with it, a note on the things that can move it, and a suggested recheck at the retest. It does not arrive with a diagnosis, because a diagnosis needs a repeat test, antibodies, and a clinician who has examined you. What it does is make sure the retest happens, which is the step that most often gets lost between one check-up and the next.

When to see a doctor

• TSH above 10 mIU/L on any reading.
• TSH raised on two readings three months apart.
• Any abnormal TSH with a free T4 outside the range.
• A low TSH that persists.
• Symptoms alongside any abnormal result: fatigue, weight change, hair loss, palpitations, feeling cold or hot, or changes to periods.
• Pregnancy or planning one, where thresholds are lower and the stakes higher.

The laboratory has its own procedure for anything urgent and will contact you under it. Everything short of that is a matter of reading the three together, twice, before deciding anything.