Supplements to mention before a blood test

Why this matters

Thyroid blood tests - including TSH and free T4 - are sensitive measurements. A handful of common supplements can either skew the result itself or affect how the lab analyses the sample. The fix is usually straightforward: pause the supplement for a short time before the test, or simply tell the lab what you're taking so they can account for it. Always check with your clinician or the lab first - never stop a prescribed medication without advice.

Biotin (vitamin B7)

Biotin is the supplement most consistently flagged by labs in relation to thyroid testing. Many thyroid tests use immunoassays, and high-dose biotin in the blood can interfere with them - distorting the result and, if not recognised, leading to confusion or unnecessary investigation.

A standard multivitamin poses no risk, but for high-dose supplements (often sold for hair or nail health) NHS labs advise not taking biotin close to the test. Gloucestershire Hospitals, for example, advises that for high-dose biotin (more than 5 mg a day), the sample is collected at least 8 hours after your last dose - and some labs advise stopping for a day or more before a non-urgent test. Check with your lab or GP what they recommend, as the exact advice depends on the assay they use and the dose you take.

Iodine and kelp supplements

High-dose iodine or kelp supplements can directly affect thyroid hormone levels, not just test readings. If you have been taking these, your clinician needs to know - both for interpreting your results and for your overall care. See iodine: not more-is-better for more on why these supplements warrant caution.

Iron supplements

Iron supplementation does not directly interfere with thyroid test accuracy, but iron deficiency and thyroid conditions can overlap, and iron affects how the thyroid functions. More practically: if you are taking iron supplements and levothyroxine, the timing of these relative to your medication matters - see food & medication timing.

Calcium supplements

Like iron, calcium does not typically skew the thyroid test result itself, but high calcium levels in the blood (hypercalcaemia) can occasionally be a clinical finding that your clinician needs context for. And again, calcium affects levothyroxine absorption - worth flagging if you take both.

Selenium

Selenium is often discussed in online thyroid communities. The evidence around selenium supplementation in thyroid conditions is mixed, and it is not something to self-prescribe - high doses of selenium are harmful. If you are taking it, mention it to your clinician so they have the full picture of what you are taking.

Vitamin D

Vitamin D deficiency is common in the UK and often co-occurs with thyroid conditions. If you are taking vitamin D supplements - especially in higher doses - mention it at your appointment. Vitamin D levels are sometimes checked alongside thyroid tests, and knowing your supplement dose helps interpret the result.

What to do before your next blood test

• Write down all the supplements you take, including dose and brand.
• Mention them when you book or arrive for your blood test - the lab or phlebotomist can note this on your sample.
• Ask your GP's surgery or the lab specifically about biotin if you take a hair/nail supplement.
• Never stop a prescribed medication before a blood test without speaking to your doctor first.
• Use the lab results tool to track your results over time.

Questions to raise with your clinician

• Do any of my supplements affect my thyroid blood test results?
• Should I pause anything before my next test, and for how long?
• Can you check whether my lab's method is affected by biotin?