Thyroid: which tests tell you what, and how to do them properly

The thyroid is checked with a single test, TSH. Others follow only if needed. Most wrong results are not made in the lab but before it. The usual causes: a biotin supplement, the wrong time of the draw, or a tablet taken just before the draw. Below is what to order, when and how, and what each combination of values means.

What the thyroid does

The thyroid is a small gland below the larynx that regulates the speed of metabolism. It affects heart rate, body temperature, digestion, weight, mood, sleep, skin, hair and, in women, the menstrual cycle.

It works in a feedback loop with the brain. The pituitary gland releases TSH, which is an order to the thyroid. In response, the thyroid releases the hormones T4 and T3. When there is enough hormone, the brain reduces TSH. When there is too little, it increases it.

Signs that people blame on something else

Underactive thyroid (hypothyroidism)

Tiredness that rest does not fix · feeling cold when others are not · constipation · dry skin and brittle hair · weight gain without a change in diet · slowed thinking and poorer memory · low mood · heavier or more irregular periods · slower heart rate · puffiness around the eyes.

Overactive thyroid (hyperthyroidism)

A pounding heart or a feeling of skipped beats · trembling hands · restlessness and anxiety · insomnia · sweating and poor tolerance of heat · weight loss despite a normal appetite · more frequent bowel movements · muscle weakness, especially in the thighs.

It is worth checking when several of these signs occur together. Other reasons to check: unexplained tiredness lasting several weeks, autoimmune disease in the family, and the time after childbirth. Also when planning or during pregnancy, and with a palpable lump in the neck.

Which tests exist and when to use which

A common mistake is to order everything at once. Thyroid tests are a hierarchy, not a list.

1. TSH — the first and often the only test needed

TSH is the screening test. In most laboratories the reference range is approx. 0.4–4.0 mIU/L, but it differs between laboratories. The range on your own report always applies. The upper limit naturally shifts upward with age. That is why the professional literature recommends age-adjusted ranges, to avoid misdiagnosis in older people.

2. Free T4 (fT4) — the second step

It is ordered when TSH is outside the range. Together with TSH it tells you where the problem is:

3. Free T3 (fT3) — less often than people think

It is useful mainly when hyperthyroidism is suspected. In hypothyroidism, or for monitoring levothyroxine treatment, it rarely changes the decision on its own. During an acute illness it is often low without the thyroid being diseased.

4. Anti-TPO and anti-Tg antibodies — the "why" question

They do not measure function but the cause. Raised levels point to an autoimmune process, most often Hashimoto's thyroiditis. Anti-TPO is the most useful single test for autoimmunity; anti-Tg complements it. They are measured once, to answer the question "why". Repeated measurement to monitor the disease is not needed. The antibody level does not tell you how severe the disease is or how well it is controlled.

5. TRAb, ultrasound and thyroglobulin — only for a specific question

TRAb is measured in hyperthyroidism to distinguish Graves' disease from other causes. Ultrasound makes sense with a palpable lump, visible enlargement or suspected nodules. It is not a screening test. In a person without signs it often finds unimportant nodules. That triggers a chain of further tests with no benefit. Thyroglobulin is used for follow-up after thyroid cancer treatment, not to assess function.

How to do the test properly

This is the most important part. Most misleading thyroid results do not come from the laboratory but from the circumstances of the draw.

Stop biotin at least 2 days before

Biotin (vitamin B7) is common in supplements "for hair, skin and nails" and in B-complexes. It interferes with most modern thyroid tests, which use biotin–streptavidin binding. The result is a typical pattern: falsely raised fT4 and fT3 and falsely low TSH. The report looks like hyperthyroidism in a healthy thyroid. Even a single 10 mg dose can distort the measurement. The American Thyroid Association recommends stopping it at least 2 days before the draw, and longer with high doses.

In the morning, and always at roughly the same time

TSH has a daily rhythm. It is highest early in the morning, falls during the day and is lowest around midday. The difference is not small. The same person can have TSH above the upper limit in the morning and within range in the afternoon. In practice: draw between 7 and 9 am, and at the same time for repeats.

Take levothyroxine after the draw

A tablet taken a few hours before the draw temporarily raises the measured fT4 and gives the impression of too high a dose.

Do not test during an acute illness

During an infection, after surgery or under marked strain, the values are often altered without the thyroid being diseased (non-thyroidal illness syndrome). Wait at least 4–6 weeks after recovery.

Same laboratory, if you care about the trend

Different analyzers give slightly different values for the same sample and have different reference ranges. Otherwise a "change" can be caused by switching laboratories, not by your body.

Do not make a diagnosis from a single report

A TSH deviation is often temporary. Before concluding there is a lasting disorder, the measurement is repeated. This is usually done after 6–12 weeks, together with fT4 and antibodies. During pregnancy, different reference ranges apply by trimester. Do not compare a pregnancy report with the usual range.

Subclinical hypothyroidism: raised TSH with normal fT4

This is the most common borderline result. It is also the most common source of unnecessary worry — or, the other way round, of an overlooked problem. What the professional societies say about it:

• TSH above 10 mIU/L: treatment is generally recommended.
• TSH below 10 mIU/L: the decision is individual. The American societies consider symptoms, the presence of anti-TPO and cardiovascular burden. The European association suggests a trial of treatment in people under 65 with symptoms.
• Over 65: treatment generally only with persistent TSH above 10. At 7–10 the decision is tailored to the individual, taking the heart and bones into account.

So a raised TSH on its own is not a diagnosis and not an automatic reason for a tablet. What matters is whether it persists on repeat, whether you have symptoms, whether the antibodies are positive and how old you are.

What not to order

Some tests are sold as "in-depth thyroid diagnostics" but give no useful answer here. Reverse T3 has no established role in routine care. Urine iodine does not assess deficiency in an individual; it fluctuates strongly and is useful for population studies. IgG antibodies to foods are advised against by professional societies. And repeated antibody measurement to monitor an already established diagnosis is not useful.

Frequently asked questions

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