Supplements and Thyroid Medicines: Absorption and Test Results

The question

Thyroid hormone medicine (levothyroxine) is taken by many people for an underactive thyroid. Two different things can go wrong with supplements: they can change how much medicine the body takes in, and they can change the blood test used to decide the medicine amount. A third issue is that a few herbs are specifically cautioned for people with thyroid conditions. This page keeps those three apart.

1. Absorption: minerals and some foods

A 2017 review in Clinical Therapeutics describes levothyroxine as a drug with a narrow therapeutic index, given in tiny amounts (micrograms), which makes absorption interactions clinically significant. [1] It reports that a decrease in absorption is "established and clinically significant" when levothyroxine is taken together with, among others, calcium carbonate, calcium citrate, calcium acetate and iron sulfate. [1] It also found that soybeans and coffee had the greatest effect on reducing absorption among foods and drinks reviewed. [1] The review says these interactions can be avoided by separating the two products in time. [1]

Two official medicine pages say the same in their own words:

• MedlinePlus (US National Library of Medicine) gives a specific spacing for calcium carbonate and iron supplements: at least 4 hours before or after levothyroxine. [2] That is a label statement for that product, not advice for any one person; ask the pharmacist what applies to you.


• The NHS (UK) lists calcium and iron supplements among products to tell the doctor about, along with St John's wort. [3] It also says some foods can affect how levothyroxine works. [3]

Label: documented (review of published studies and official medicine information). For the minerals themselves, see Calcium, Iron and Mineral Interactions and Absorption.

2. Lab-test interference: biotin

Biotin (vitamin B7) is a vitamin sold in supplements. The US Food and Drug Administration (FDA) says biotin in dietary supplements "can significantly interfere with certain lab tests and cause incorrect test results which may go undetected." [4] The NHS names biotin on the same list for levothyroxine users because it can affect thyroid tests. [3]

A published case report describes a patient whose blood tests suggested an overactive thyroid; after biotin was stopped and the labs repeated a month later, the TSH result had normalized. [5] A single case report cannot show how often this happens, but it shows why the lab and prescriber should know about biotin before a test.

Label: documented as a regulator warning; the thyroid example is a case report. Read more: Biotin and Lab-Test Interference.

3. Herbs cautioned for thyroid conditions

NCCIH says ashwagandha (Withania somnifera, accepted name per GBIF [6]) "is not recommended for people who are about to have surgery, or for those who have autoimmune or thyroid disorders," and that it might interact with several medicine groups, including thyroid hormone medicines. [7] NCCIH also notes that a number of cases link liver injury to ashwagandha supplements, which is rare. [7] More detail: Ashwagandha and the Thyroid.

The NHS also lists the herbal supplement St John's wort among things that can affect levothyroxine. [3] The sources we read for this page do not explain the mechanism.

Limits

Proper evidence not found, in the sources we read, for seaweed, kelp or iodine supplements and their effect on thyroid medicine amounts; see Seaweed, Iodine and the Thyroid for what that page covers. Other herbs may matter, but "not described in the sources we read" is not the same as "safe."

What a reader can do

• Tell the prescriber and pharmacist about every supplement, including biotin, calcium and iron, before thyroid blood tests and when anything changes.
• Mention biotin to whoever orders or runs the blood test, not only to the prescriber.
• Do not stop, skip or change thyroid medicine because of a supplement; ask the prescriber or pharmacist.
• Because levothyroxine has a narrow range, ask before adding anything new.
• Background: How Interactions Work and Thyroid Problems and Fatigue.