Thyroid function tests explained: TSH, free T4 and T3

What the thyroid does and what the tests measure

The thyroid is a small gland at the front of the neck. It makes hormones that help set the pace of the body’s metabolism, affecting heart rate, energy use, body temperature, digestion and mood. The brain controls it through a feedback loop: the pituitary gland, at the base of the brain, releases thyroid-stimulating hormone (TSH) when it senses that thyroid hormone levels are too low, and releases less when they are high.

That feedback loop is why TSH is the usual first test. A small change in thyroid hormone produces a larger change in TSH, which makes it a sensitive signal. A typical thyroid profile on an Indian report lists:

• TSH: the pituitary’s signal to the thyroid.
• Free T4 (FT4): the unbound, active form of thyroxine, the main hormone the thyroid makes.
• Total T4 and total T3 on some reports: they include hormone attached to carrier proteins as well as the free part.
• Free T3 (FT3) or total T3: triiodothyronine, the more active hormone that T4 is converted into.
• Anti-TPO (thyroid peroxidase) antibodies, when a doctor is looking for an autoimmune cause.

Units on a thyroid report

TSH is reported in milli-international units per litre (mIU/L) or, equivalently, micro-international units per millilitre (µIU/mL). These two units give exactly the same number, so a TSH of 2.5 mIU/L is also 2.5 µIU/mL.

Free T4 is often printed in nanograms per decilitre (ng/dL) in India and picomoles per litre (pmol/L) in many other countries. To convert, multiply ng/dL by 12.87: for example, 1.2 ng/dL is about 15.4 pmol/L. Total T4 in micrograms per decilitre (µg/dL) converts to nanomoles per litre (nmol/L) with the same factor, 12.87.

Reference ranges differ by laboratory and method. A TSH range printed as 0.4 to 4.0 or 0.45 to 4.5 mIU/L is common, but you may see slightly different limits on your report. Use the range printed beside your own result.

How the three results are read together

A doctor reads TSH with free T4 (and sometimes T3), never TSH in isolation. Some broad patterns, in simplified form:

“Subclinical” patterns are common. Doctors frequently repeat the test after several weeks before deciding what, if anything, to do, because TSH naturally fluctuates and a transient rise can follow an illness. Whether to treat a mild abnormality depends on the level, symptoms, age, pregnancy plans, antibody results and other conditions.

Symptoms of thyroid conditions, such as tiredness, weight changes, feeling too cold or too hot, a racing heart or constipation, are also common in people with normal thyroid function. That is why symptoms plus tests together matter, and why a thyroid test is not a stand-alone answer.

Why timing matters

• Time of day: TSH follows a daily rhythm, usually higher late at night and in the early morning and lower in the afternoon. Doctors trying to compare values over time often like to test at a similar time of day.
• Recent illness: a significant illness can temporarily change thyroid results without the thyroid itself being diseased. Doctors often avoid labelling a thyroid problem during or just after a serious illness.
• Pregnancy: thyroid hormone needs and results change through pregnancy, and many laboratories use trimester-specific ranges. Pregnant women should be assessed against pregnancy ranges, not the standard adult range.
• Infancy, childhood and older age have their own reference ranges, too.

If you take thyroid tablets, your doctor may repeat tests about six to eight weeks after any change, since it takes time for levels to settle. Please do not change your tablet amount on the basis of a result: only the prescribing doctor should do that.

Medicines and supplements that matter

Several things can affect either the thyroid itself or the test:

• Levothyroxine absorption: iron and calcium supplements, antacids and some other medicines can reduce how much thyroid tablet your body absorbs if taken too close together. Ask your doctor or pharmacist how far apart to take them.
• Biotin (vitamin B7): this supplement, often present in hair, skin and nail products and in multivitamins, can interfere with some laboratory immunoassays and cause falsely abnormal thyroid results. Tell the laboratory and your doctor if you take it, and ask whether you should pause it before testing.
• Amiodarone, lithium, some cancer treatments and iodine-containing contrast dye or supplements can alter thyroid function.
• Steroids, dopamine-affecting drugs, oestrogen-containing medicines and others can change thyroid hormone levels in the blood.

On the morning of a test, ask your doctor whether you should take your thyroid tablet before or after the blood sample. Practice varies, and the key is to be consistent from one test to the next.

A worked example (example values)

Take two example reports for the same person, taken ten weeks apart: first TSH 7.8 mIU/L with free T4 1.1 ng/dL, then TSH 5.4 mIU/L with free T4 1.0 ng/dL, with a laboratory range for TSH of 0.4 to 4.2 mIU/L.

Both TSH values are above the printed range while free T4 stays within its range, which fits the pattern often called subclinical hypothyroidism. The second value is lower than the first, and the person had a viral illness before the first test. A doctor reading these might repeat the test again, check antibodies, and ask about symptoms, family history and pregnancy plans before deciding whether to treat. Another doctor might recommend treatment after the first result. Both could be reasonable. It shows why the decision belongs in a conversation, not in a single flag.

Converting the example free T4 from ng/dL to pmol/L gives about 14.2 and 12.9 pmol/L. If the second report had been printed in pmol/L while the first used ng/dL, comparing 12.9 with 1.1 directly would look like a huge fall, when nothing had changed. Always check the unit.

Antibodies, scans and what happens next

If thyroid tests are abnormal, doctors may ask for further information before deciding what to do.

• Anti-TPO and anti-thyroglobulin antibodies show whether the immune system is attacking the thyroid, which is the commonest cause of an underactive thyroid in adults. Positive antibodies can be present for years with normal hormone levels, so a positive result does not mean treatment is needed at once.
• TSH receptor antibodies help to identify the cause of an overactive thyroid.
• An ultrasound scan of the neck looks at the size and texture of the gland and any lumps. A lump or swelling in the neck is assessed separately from blood hormone levels, and normal blood tests do not rule out the need to examine a lump.
• A radioactive iodine uptake scan is sometimes used to find the cause of an overactive thyroid.

Not everyone needs all of these. The choice depends on your symptoms, results and age, and on what the doctor suspects.

Thyroid tests and pregnancy planning

Thyroid hormone matters for the developing baby, so doctors often check thyroid function in women who are planning a pregnancy or are newly pregnant, particularly when there is a history of thyroid disease, antibodies or a family history. Targets for TSH in pregnancy differ from the usual adult range, and women already taking thyroid tablets are usually advised to tell their doctor as soon as they know they are pregnant, because the need for treatment can change. Do not adjust tablets yourself: contact the prescribing doctor promptly.

Questions worth asking your doctor

• Are my results a one-off, or do we need to repeat them?
• Do I need an antibody test or a scan?
• Could my medicines or supplements, especially biotin, have affected the result?
• Should I take my thyroid tablet before or after the sample next time?
• How soon should I be retested, and what would change the plan?

Sources

• American Thyroid Association: Thyroid Function Tests — patient overview from a specialist society
• MedlinePlus: TSH (Thyroid-Stimulating Hormone) Test — how the test is used and read
• MedlinePlus: Thyroxine (T4) Test — total and free T4
• MedlinePlus: Triiodothyronine (T3) Tests — when T3 is measured
• NHS: Underactive thyroid (hypothyroidism) — symptoms, tests and treatment overview

A note on this guide

This guide is general information about thyroid reports. It is not medical advice, and it cannot tell you whether you have a thyroid condition. Confirm the meaning of your results, and any treatment, with your doctor.