Thyroid function tests: TSH, T3, T4 and antibody tests explained

A simple blood test is often the first step when a doctor thinks your thyroid might not be working as it should. This article explains the main thyroid tests and what they are used for, using information from MedlinePlus, the NIDDK and NHS. It cannot tell you what your own results mean. Only your doctor can do that.

What do thyroid tests check?

MedlinePlus explains that thyroid tests check how well the thyroid, a butterfly-shaped gland at the front of the neck, is working. Doctors use them to diagnose thyroid diseases such as hyperthyroidism and hypothyroidism and to help find their cause. The thyroid makes hormones that control the way the body uses energy, and they affect nearly every organ.

Blood tests

• TSH (thyroid-stimulating hormone): MedlinePlus says this is usually the first test a provider orders.
• T4 (thyroxine) and T3 (triiodothyronine): these measure the levels of thyroid hormones in the blood.
• Thyroid antibody tests: these measure certain antibodies and may help diagnose autoimmune thyroid disorders. MedlinePlus names thyroid peroxidase and antithyroglobulin antibody tests. For Hashimoto's disease, the NIDDK says doctors may test for thyroid peroxidase antibodies. For Graves' disease, the NIDDK says doctors test thyroid hormone levels and an antibody called thyroid-stimulating immunoglobulin.

The NHS says that for an underactive thyroid, blood tests measure TSH and T4. For an overactive thyroid, it says a blood test checks how well the thyroid is working, and further tests may find the cause.

Imaging and other tests

MedlinePlus lists several imaging tests, including:

• Thyroid scan: shows the size, shape and position of the thyroid and any nodules
• Radioactive iodine uptake test: a nuclear medicine test
• Thyroid ultrasound: the NIDDK says a Doppler ultrasound is often used for Graves' disease when radioiodine is not appropriate, for example in pregnancy or breastfeeding
• Needle biopsy: MedlinePlus lists needle and ultrasound-guided biopsies among tests used in some cases

The NHS says for a goitre, doctors usually use blood tests, an ultrasound scan, or a needle biopsy of the thyroid to find the cause.

Why symptoms are not enough

The NIDDK says that for both hypothyroidism and hyperthyroidism, symptoms alone cannot confirm the diagnosis, because they overlap with many other conditions. Fatigue and weight gain, for instance, are common and do not necessarily point to the thyroid. This is why a test is needed.

Monitoring after diagnosis

For underactive thyroid treated with thyroid hormone medicine, the NIDDK says a blood test is done about 6 to 8 weeks after starting and after each dose change, then roughly every 6 months, and yearly once the dose is stable. Your doctor decides the schedule. Never change your medicine on the basis of a test result without medical advice.

Preparing for a test

The health pages reviewed here do not list specific preparation steps for thyroid tests. Ask your doctor or the testing lab whether anything is needed, and tell them about every medicine and supplement you take, since some contain iodine. The NIDDK notes that iodine-containing medicines and supplements may affect hyperthyroid and Hashimoto's conditions.

When to see a doctor

Ask a doctor about thyroid tests if you have symptoms such as tiredness, a change in weight, feeling too cold or too hot, a fast or slow heartbeat or a swelling in the neck. The NIDDK also advises getting tested for hypothyroidism if you have high cholesterol. Women who have trouble conceiving are often tested for thyroid problems.

Seek emergency help for trouble breathing, a wheezing noise when breathing, a very fast irregular heartbeat, chest pain or confusion. In an emergency, call your local emergency number (112 in India, 911 in the United States).

This article is for education only and does not replace advice from your own doctor.