Thyroid Conditions: Iodine, Biotin and Herbal Products

Thyroid health depends on two things that supplements can disturb: the amount of iodine the thyroid receives, and the blood tests used to check how it is working. This page sets out what the sources we read say about iodine and kelp, biotin and lab tests, and a few herbal products. It is information only. Thyroid medicine is prescribed to a person, so ask the prescriber or pharmacist before adding or changing anything.

Iodine and kelp

MedlinePlus says people need iodine for normal thyroid function and for making thyroid hormones. [1] Both too little and too much can matter. Kelp, a seaweed, gets special attention because of the iodine it can supply. Our main page on iodine covers food sources; the page on seaweed, sea moss and kelp covers the seaweed products themselves.

That last row is a good example of why we label evidence. A caution can be sensible even when no case has been published, and the lack of reports is not proof of safety. The same reference says kelp products should be avoided in people with thyroid disorders. [2]

Biotin and thyroid blood tests

Biotin (vitamin B7) does not treat the thyroid. The concern is that it can change the test result. Excess biotin in a blood sample can interfere with a type of test called an immunoassay. [3]

• In 2017 the U.S. FDA warned that biotin can "significantly interfere with certain lab tests and cause incorrect test results." [3]
• Singapore's Health Sciences Authority (2019) describes biotin interference causing a falsely low TSH and falsely high free T3 and free T4, which it says could result in a misdiagnosis of Graves' disease. [4]
• The same authority lists people who have a higher risk of wrong results, including people with kidney failure, newborns, children and pregnant women. [4]
• FDA's reports included one death, linked to a falsely low result on a different test, the troponin test used for heart attacks. [3]

Biotin is easy to miss. Vitamins sold for hair, nails and skin may list it only on the back label in small print. [3] The practical step the sources give is to tell your clinician, and the lab, that you take biotin. [3] For more, see biotin and lab-test interference.

Herbal products and thyroid medicine

• Ashwagandha (Withania somnifera). NIH's NCCIH says ashwagandha is not recommended for people who are about to have surgery or who have autoimmune or thyroid disorders, and that it might interact with several medicine groups, including thyroid hormone medicines. [5] See ashwagandha and the thyroid.
• Levothyroxine and the label. MedlinePlus tells people taking levothyroxine to give the doctor and pharmacist a list of all prescription and nonprescription medicines, vitamins, supplements and herbal products. [6] It also names some nonprescription products that may interact. [6] Our page on supplements and thyroid medicines goes further.
• Weight loss. MedlinePlus says levothyroxine should not be used alone or with other treatments to treat obesity or cause weight loss. [6] See eating disorders and weight-loss supplements.

"Proper evidence not found": we looked for statements about "thyroid support" blend products in the NCCIH, MedlinePlus and clinical-reference pages above and found none, so we cannot say what any blend contains. Check the label for kelp, iodine, biotin and ashwagandha, and see reading supplement labels. We could not open the NIH Office of Dietary Supplements iodine and biotin fact sheets in this session, so no upper limits are quoted here.

Who should ask first

Anyone who takes thyroid medicine, has Hashimoto's or Graves' disease, is pregnant or planning pregnancy, or is having thyroid blood tests soon should show their pharmacist or clinician every supplement. A pharmacist can read the label with you. See talking with a health professional and how interactions work.