Iodine (kelp, potassium iodide)

What it is

Iodine is a trace mineral the thyroid gland needs to make hormones that regulate metabolism, growth and brain development. Good sources are white fish, shellfish, dairy and some seaweed. Iodised salt is less common in the UK than in some countries.

UK surveys have shown low iodine intakes in some groups, particularly teenage girls, young women and people who follow plant-based diets without iodine-fortified products. Iodine matters most in pregnancy and breastfeeding because of its role in the baby's brain development.

Supplements come as potassium iodide or kelp. Kelp products are unpredictable, and some deliver very high amounts.

How it works

The thyroid takes iodine from blood and builds thyroxine (T4) and triiodothyronine (T3). Too little iodine causes the gland to enlarge (goitre) and hormone levels to fall. Too much iodine can paradoxically cause both underactive and overactive thyroid, especially in people with existing thyroid disease.

What the evidence says

• Strong Correcting iodine deficiency
  Prevents and reverses goitre and low thyroid function caused by lack of iodine.
• Moderate Brain development in pregnancy
  Adequate iodine is important; moderate deficiency appears linked with poorer child outcomes, but supplementing already-adequate women gives unclear benefit.
• Weak General energy or weight loss
  No good evidence that extra iodine boosts metabolism in people with normal thyroids.
• Strong Thyroid protection in nuclear emergencies
  Specific potassium iodide tablets are issued by authorities in emergencies only; not for routine use.

Evidence ratings are our plain-English summary of how much good human research exists, not a promise of results. How we rate evidence.

Dosage and timing

• Many multivitamin and prenatal supplements provide about 150 mcg, matching typical needs.
• UK guidance has suggested long-term supplemental intakes around 500 mcg or less per day are unlikely to cause harm in healthy adults; check current guidance.
• Kelp tablets can vary a lot, so look for a stated microgram amount.
• Do not self-medicate for thyroid symptoms.

Forms and what to look for

• Potassium iodide or iodate: precise, standardised doses.
• Kelp or seaweed: natural but iodine content varies widely and can be very high.
• Multivitamin or prenatal formulas: often contain a safe, modest amount.
• Avoid high-dose iodine solutions like Lugol's unless prescribed.

What to expect, and when

• You will probably not feel any difference if your intake was adequate.
• If deficient, thyroid function can improve over weeks to months.
• Symptoms of excess, such as palpitations, can appear within days to weeks.

Side effects

• Metallic taste, stomach upset
• Skin reactions or acne-like rash
• Thyroid overactivity (palpitations, anxiety, weight loss, tremor) with excess
• Underactive thyroid (tiredness, weight gain) with excess in susceptible people

Interactions

• Interferes with antithyroid drugs (carbimazole) and levothyroxine
• Can interact with lithium
• Amiodarone contains a lot of iodine and affects the thyroid
• Large doses combined with ACE inhibitors or potassium-sparing diuretics can raise potassium

Who should avoid it

• Anyone with thyroid disease (Hashimoto's, Graves', nodules) unless advised by their doctor
• People taking thyroid medication or lithium
• Anyone using kelp or seaweed in large amounts without knowing the iodine content
• Pregnant women taking more than recommended; follow midwife advice

Risks and safety

• Kelp supplements have been found to contain very high or inconsistent iodine amounts, and sometimes arsenic or other heavy metals.
• Excess iodine can trigger thyroid dysfunction.
• Not on WADA list.
• Hijiki seaweed is not advised because of arsenic.

Buying well: quality and testing

• Choose products that state micrograms of iodine per dose.
• Look for third-party testing for heavy metals, especially for kelp.
• Avoid products promoting thyroid cures.
• Prefer standardised potassium iodide to raw seaweed.

Bottom line

If you eat little fish, dairy or fortified products, or you are pregnant or planning to be, a modest iodine supplement may make sense. Do not take high-dose kelp for the thyroid, and talk to a clinician first if you have thyroid disease.