Reading labels when you have a thyroid condition

A thyroid is an unusual thing to have to read labels for, because the ingredients that matter are not sold for thyroids and say nothing about them.

Three of them come up repeatedly, and all three are ordinary.

Biotin, which distorts the test rather than the gland

This is the one almost nobody knows and the one with the largest consequence.

Many blood tests are built on the binding between biotin and streptavidin. Flooding a sample with extra biotin competes with that chemistry. The 2026 European Thyroid Association guideline records that biotin, “widely used as a supplement”, has “been described to cause falsely high or low hormone results in streptavidin-biotin-based assays”.

The NHS puts the same point in its patient information for levothyroxine, listing “biotin (vitamin B7) as it can affect your thyroid tests” among things that do not mix well.

Nothing is wrong with you. The number is wrong. And a result showing suppressed TSH with raised free hormones, which is what this can produce, looks like Graves’ disease.

Why it catches people: biotin is the main ingredient in hair, skin and nails supplements, taken for reasons that have nothing to do with a thyroid. Nobody thinks to mention a hair vitamin at an endocrinology appointment.

The guideline’s recommendation where medication interference is suspected is to repeat the analysis “after omission of the suspected agent, if possible”. That is a conversation, not a decision to make alone. The full account is on Biotin.

Calcium and iron, which block absorption

The NHS lists “calcium and iron supplements” and “antacids and other indigestion treatments” among the things that may not mix well with levothyroxine.

The mechanism is absorption rather than interference. These bind levothyroxine in the gut so less of it reaches you, which shows up as a dose that has stopped working rather than as anything dramatic.

Timing is the usual answer and it belongs to your doctor or pharmacist rather than to this page. What a label can do is tell you that a multivitamin contains Calcium carbonate or Ferrous sulfate, which is often not obvious from the front of a pack.

The NHS also advises taking levothyroxine “30 to 60 minutes before your first meal of the day, or any drink with caffeine, such as tea or coffee”. Caffeine is on this page for that reason and no other.

Iodine, which acts on the gland itself

The thyroid uses iodine to make its hormones, and both too little and too much cause problems.

The NHS puts adult need at “140 micrograms (μg) of iodine a day” and is direct about the upper end: “Taking high doses of iodine for long periods of time could change the way your thyroid gland works.” It gives a practical threshold: “Taking 0.5mg or less a day of iodine supplements is unlikely to cause any harm.”

Between those two numbers is a factor of about three and a half, and seaweed supplements can carry far more than either. Kelp is the common route to an accidental large dose, and the amount in a seaweed product is frequently not stated in a form you can use.

Iodine has the detail, including why a vegan diet is the common route to the opposite problem.

The one sentence worth carrying

If you are having thyroid bloods taken, tell whoever is taking them what supplements you are on, including the ones sold for hair and nails.

It costs nothing, it takes five seconds, and it prevents the specific chain this page exists to describe: a normal thyroid producing an abnormal number, followed by repeat tests, a scan, a referral, and occasionally treatment for a condition that was never there.