Thyroid function tests: what is included

In shortThyroid function tests usually start with TSH, the most sensitive single measurement. Free T4, total or free T3, and thyroid antibodies are added to clarify what an abnormal TSH means. Biotin supplements can distort the results.

What this panel covers

Thyroid tests measure how well the thyroid gland is working. The gland sits in the front of the neck and makes two main hormones, T4 and T3, which control how the body uses energy. The pituitary gland in the brain makes TSH — thyroid stimulating hormone — to tell the thyroid how much T4 and T3 to produce.

TSH is usually the first test ordered, because it is the best initial test for thyroid function: when the thyroid underproduces, the pituitary raises TSH to push it harder, and when the thyroid overproduces, TSH falls. For screening in people without symptoms, TSH alone may be sufficient.

The follow-up tests explain an abnormal TSH. Free T4 measures the active, unbound form of the main thyroid hormone; T3 tests measure the other thyroid hormone, either total or free; and antibody tests — for thyroid peroxidase, thyroglobulin, or TSH receptor antibodies — look for an autoimmune cause such as Hashimoto disease or Graves disease.

One practical warning applies to all of these: biotin — vitamin B7, a common ingredient in hair, skin, and nail supplements — can interfere with thyroid test measurements and produce misleading results. Supplements containing biotin are typically paused for a few days before the blood draw, on the provider's advice.

Markers on the panel

TSH (thyrotropin)Free T4 Free T3 TPO antibodies Anti-thyroglobulin antibodies TSH receptor antibodies Thyroglobulin Calcitonin

Why it is ordered

To check how well the thyroid gland is working: to find an underactive thyroid (hypothyroidism) or an overactive one (hyperthyroidism), to follow up symptoms such as unexplained weight change, fatigue, or heart rate changes, and to monitor treatment with thyroid medicines.

How to prepare

Usually no special preparation is needed. Biotin (vitamin B7) in supplements can distort the measurements, so all supplements should be reported — they are commonly paused for a few days before the draw on the provider's advice.

How to read the report

The tests are read as a ladder. TSH comes first: a high TSH with low free T4 means an underactive thyroid; a low TSH with high free T4 or T3 means an overactive one. When TSH is abnormal but free T4 is still in range, the pattern is called subclinical. Antibody results then answer the question of cause — whether the immune system is attacking the gland.

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Frequently asked questions

Why is TSH usually the first test?

Because it is the most sensitive single measurement of thyroid function: the pituitary reacts to small changes in thyroid hormone levels by adjusting TSH, so TSH shifts before T4 and T3 leave their ranges.

Can supplements affect the results?

Yes. Biotin (vitamin B7), common in hair and nail supplements, can interfere with thyroid measurements and give misleading results. Providers typically ask for biotin supplements to be paused for a few days before testing.

What happens after an abnormal TSH?

Usually free T4 is measured next to confirm and grade the change; T3 and antibody tests follow when needed to find the cause, such as Hashimoto disease or Graves disease.

Sources

Thyroid Tests: MedlinePlus Medical Encyclopedia — MedlinePlus, National Library of Medicine. Accessed: August 29, 2026.
Thyroid Tests: MedlinePlus Health Topic — MedlinePlus, National Library of Medicine. Accessed: August 29, 2026.