Thyroid function test explained: how to read your TSH, T4 and T3 report

The thyroid test is one of the most common blood tests done in India. Thyroid problems are common here: a study across eight Indian cities found that about one in ten adults had an underactive thyroid, and many did not know it. The good news is that the report is not hard to understand once you know what each line means. You can also enter your own numbers into the thyroid report reader to see which pattern they fit.

What does the thyroid gland do?

The thyroid is a small, butterfly-shaped gland in the front of your neck. It makes thyroid hormone, which sets the speed of your body’s metabolism. It affects your energy, heart rate, weight, bowel habits, skin, hair, periods and mood.

The thyroid does not work alone. A pea-sized gland at the base of the brain, called the pituitary, keeps watch on thyroid hormone levels. When it senses too little hormone, it sends out more TSH (thyroid stimulating hormone) to push the thyroid to work harder. When there is too much hormone, it turns TSH down.

What does TSH tell you?

TSH is the most useful single thyroid test. Because the pituitary reacts to small changes, TSH often moves before the thyroid hormones do.

• High TSH usually means the thyroid is underactive and the pituitary is shouting at it to work harder. This is hypothyroidism.
• Low TSH usually means there is too much thyroid hormone, so the pituitary has gone quiet. This is hyperthyroidism, or too much thyroid medicine.

Indian labs print TSH in µIU/mL or mIU/L. These are the same number, just written differently.

What are T4, FT4, T3 and FT3?

The thyroid mainly releases T4 (thyroxine). The body converts some of it into T3, the more active form. Most of the hormone in blood is carried on proteins. Only a small part is “free” and able to act.

• Total T4 and total T3 measure both the carried and free hormone. Many Indian lab packages labelled “T3, T4, TSH” report these totals. Totals change when carrier proteins change, for example in pregnancy or with the contraceptive pill.
• Free T4 (FT4) and free T3 (FT3) measure only the active, unbound hormone. FT4 is usually the more reliable number to read alongside TSH.

Common units you will see are ng/dL for FT4, pg/mL for FT3, µg/dL for total T4 and ng/dL for total T3. Do not compare a number from one lab with the range from another, because methods differ.

What are thyroid antibodies?

The most common cause of thyroid problems is the immune system attacking the gland. Two antibody tests help show this:

• Anti-TPO antibodies are raised in Hashimoto’s thyroiditis, the main cause of an underactive thyroid. A positive result with a normal TSH means you have a higher chance of developing hypothyroidism later, so doctors usually suggest a repeat test from time to time.
• TSH receptor antibodies (TRAb) are raised in Graves’ disease, a common cause of an overactive thyroid.

Antibodies are usually checked once to find the cause, and anti-TPO is not normally repeated. TRAb is the exception: doctors often recheck it in Graves’ disease before deciding to stop anti-thyroid medicines, and in pregnancy, because these antibodies can cross to the baby.

How do you read the common patterns?

Doctors read TSH and FT4 together. The main patterns are:

TSH	FT4	Usual name
Normal	Normal	Normal thyroid function
High	Normal	Subclinical (mild) hypothyroidism
High	Low	Overt hypothyroidism
Low	Normal	Subclinical (mild) hyperthyroidism
Low	High	Overt hyperthyroidism

Each pattern has its own page: high TSH with normal T4, high TSH with low T4, low TSH with normal T4 and low TSH with high T4. If your results are in the normal range but you still feel unwell, read normal thyroid tests but still having symptoms.

Any other combination, such as a high TSH with a high FT4, is unusual. It needs a closer look, as explained in unusual thyroid test results.

Why is the “normal range” different in every lab?

Each lab sets its range from its own machine and method, so the limits printed on your report are the ones that apply to you. A TSH that is slightly outside one lab’s range may sit inside another’s.

Normal TSH also shifts with life stage. It tends to rise a little with age, so a mildly high TSH in a person over 70 can be normal for them. In pregnancy the targets are different, which is why pregnant women are judged against pregnancy-specific ranges. See thyroid problems in pregnancy.

A single borderline value is rarely a diagnosis. Doctors usually repeat the test after some weeks before labelling anyone with a thyroid condition.

Does the time of the test matter?

Yes, a little. TSH is naturally higher at night and in the early morning, and lower in the afternoon. Some studies show TSH is slightly lower after a meal. The difference is small, but it can tip a borderline result over the line. The simple answer is to keep your tests at a similar time of day, so that one report can be fairly compared with the next. More on this in should I fast before a thyroid test.

If you are on thyroid tablets, ask your doctor whether to give the blood sample before or after that morning’s tablet, and then keep to the same routine every time.

What can give a false result?

• Biotin (vitamin B7), found in many hair, skin and nail supplements, interferes with many modern thyroid tests. It can make a normal thyroid look overactive. Tell your doctor about any supplement before testing.
• Recent illness, such as a severe infection or hospital stay, can upset results for a few weeks.
• Some medicines, including steroids, amiodarone, lithium and some cancer medicines, change thyroid results.
• Rare antibodies in the blood, such as heterophile antibodies or macro-TSH, can give a falsely high reading that does not match how you feel.

What should you ask your doctor?

• Which pattern does my report fit, and is it mild or significant?
• Should the test be repeated before deciding anything?
• Do I need antibody tests or a thyroid ultrasound?
• Could any of my medicines or supplements be affecting the result?

If your results are confusing or keep changing, an endocrinologist can help make sense of them. See who an endocrinologist is or book a consultation.