Iodine: not more-is-better

Why iodine matters

The thyroid uses iodine from food and water to make its hormones - thyroxine (T4) and triiodothyronine (T3). Without enough iodine, the thyroid can't produce what the body needs. In parts of the world where iodine is scarce in the soil and diet, deficiency is a genuine public health concern. In the UK, most people get enough through everyday foods: dairy products, fish, and eggs are the main dietary sources.

The problem with more

Online thyroid communities often frame iodine as something to 'top up' if your thyroid is struggling. The evidence doesn't support that for most people in the UK, and high-dose iodine can actually cause problems:

• In people with an already-vulnerable thyroid (including autoimmune conditions like Graves' disease or Hashimoto's), excess iodine can trigger or worsen dysfunction.
• Large amounts can temporarily suppress thyroid hormone production - an effect used deliberately in clinical settings before surgery, but not something to reproduce at home.
• Excess iodine has been associated with triggering thyroiditis in susceptible people.

Kelp and high-dose supplements

Kelp (seaweed) products and 'thyroid support' supplements frequently contain iodine levels far above the recommended daily amount. The NHS and British Thyroid Association both advise against taking iodine supplements unless a clinician has identified a deficiency and recommended them. If you are taking any supplement that lists kelp, bladderwrack, or iodine as an ingredient, it is worth mentioning this at your next appointment.

What about a low-iodine diet?

You may have read or heard about low-iodine diets. These are a clinical protocol used in specific circumstances - typically in the weeks before radioactive iodine treatment - to make the treatment as effective as possible. A low-iodine diet is not a general thyroid-health diet and is not something to follow based on a blog post or forum advice.

If you are having radioactive iodine treatment, your clinical team will give you specific, personalised guidance on what to eat and avoid, and for how long. This is the guidance to follow - not general internet advice.

Eating seafood and seaweed normally

For most people, moderate amounts of fish, shellfish and occasional seaweed in a varied diet are not a cause for concern. The issue is with concentrated supplements and repeated large servings of high-iodine foods over time. If you are unsure how your diet maps to your specific condition, a registered dietitian is the right person to ask.

Questions to ask your clinician

• Do I need my iodine levels checked?
• Are there any supplements I should stop before my next blood test or treatment?
• If I am having radioactive iodine, what exactly should I eat or avoid, and from when?

See also food & medication timing for how food and supplements can affect thyroid medication absorption, and use the questions tool to put together a list for your next appointment.