Preparing for a Thyroid Blood Test
Thyroid blood tests need very little preparation, but a few simple steps help ensure your result is accurate and easy to interpret.
This page is a practical checklist for the days and hours before your test.
Last medically reviewed: 15 November 2025
Food and drink
No fasting is usually required for thyroid tests. Eat and drink normally unless you are having other tests on the same day that do require fasting.
If you take levothyroxine, ask your clinician whether to take it before or after the blood draw — the key is to keep it consistent from test to test.
Medicines and supplements
Bring a full list of your medicines and supplements. Some — such as biotin, calcium, iron and certain prescription drugs — can affect thyroid results or absorption.
Biotin in particular can interfere with several assays, so many laboratories advise stopping it two to three days before testing.
What to bring
Bring any previous thyroid results, a list of your current medications, and details of your symptoms. This helps the clinician interpret your new result against your history.
If you are having several tests, the laboratory can usually collect everything from a single blood sample.
On the day
Wear a top with sleeves that roll up easily. Drink water so your veins are easier to find. Tell the person taking your blood if you feel faint during the procedure.
Frequently asked questions
Do I need to fast before a thyroid blood test?
No — fasting is not usually required. Only fast if other tests on the same day need it, and follow the instructions you are given.
Should I stop my supplements?
Biotin can interfere with thyroid assays, so many laboratories advise stopping it two to three days beforehand. Ask your laboratory for its specific guidance.
What should I bring to my blood test?
Bring a list of your medicines and supplements and any previous thyroid results. This helps your clinician interpret the new result accurately.
References & further reading
This page is based on established medical knowledge and guidelines from authoritative sources. Always consult a qualified clinician for personal medical advice.
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