Quick answer

Iodide from a supplement is absorbed quickly and almost completely, and the thyroid takes what it needs while most of the rest leaves in urine. Changing your iodine status takes much longer: in a trial in children with low iodine, 150 mcg a day for 28 weeks raised urinary iodine from 63 to 145 mcg/L.^1,2

Key facts

In a small study, people absorbed 96.4% of a dose of potassium iodide.¹
A healthy adult with enough iodine stores about 15 to 20 mg of it, most of it in the thyroid.¹
TSH, which drives the thyroid, can stay in the normal range even when iodine intake is low, so it is a poor guide to iodine status.¹

The first hours

Iodide is absorbed quickly and almost completely in the stomach and the first part of the small intestine. The thyroid takes up what it needs, and most of the rest leaves in urine.¹

In a radiation emergency, potassium iodide works best when taken within 3 to 4 hours of exposure.³

Potassium iodide works by flooding the thyroid with ordinary iodine, leaving no room for radioactive iodine, which then leaves in urine. An everyday supplement follows the same route on a far smaller scale.³

How long a dose lasts

A single dose of potassium iodide protects the thyroid for about 24 hours, which is why it is taken once a day for as long as a radiation risk lasts.^3,4

People excrete more than 90% of the iodine they eat in urine, so urine tests are used to check the iodine status of groups of people, though a single sample cannot tell whether one person gets enough.¹

So a single dose does not build up. The thyroid keeps what it needs and the kidneys clear most of the rest, which is why the protection from one dose of potassium iodide lasts about a day. What persists is the store held in the thyroid, which the gland draws on to keep making hormone.^1,3

How long until iodine status changes

In a trial in children with mildly low iodine, urinary iodine rose from a median of 63 to 145 mcg/L after 28 weeks of 150 mcg a day, while the placebo group stayed low at 81 mcg/L.²

In a study of six young men given a daily iodine tracer for 64 to 92 days, total body iodine averaged 14.6 mg, and full exchange of the body's iodine was predicted to take about 10 months.⁵

Urine reflects what was eaten recently, so a change in intake shows up there first, while the body's total iodine shifts over months. When intake drops below about 100 mcg a day, the pituitary raises TSH and the thyroid traps more iodine, which helps it keep up with a shortfall for a time.^1,5

What the trials measured over time

Trials of iodine supplements have run for months and followed children for years. In the trial in New Zealand children, reasoning scores were tested after 28 weeks of 150 mcg a day. In two pregnancy trials of 150 or 200 mcg a day, children were assessed at 1.5 or 2 years, and in one of them again at 5 to 6 years, with no difference in development linked to the mother's iodine.^1,2

May improve some reasoning scores in children whose iodine intake is mildly low*^1,2

Early evidence

In one randomized trial, 184 children aged 10 to 13 in New Zealand with mildly low iodine took 150 mcg a day or a placebo for 28 weeks. The iodine group scored higher on 2 of 4 reasoning tests, and its overall score was 0.19 standard deviations above placebo. It is a single trial in children who were short of iodine, so the evidence is early and says nothing about people who already get enough.

Amount studied: 150 mcg a day for 28 weeks, in children aged 10 to 13

Before radioactive iodine treatment

People preparing for radioactive iodine treatment for thyroid cancer are usually asked to eat a low iodine diet, under 50 mcg a day, for 1 to 2 weeks first, and to stop kelp and multivitamins that contain iodine. In a Dutch study of 65 patients, iodine in urine on day 4 of the diet was not significantly different from day 7, though that may not hold where diets are richer in iodine.^1,6

Ask your care team when to stop iodine supplements before a thyroid scan or treatment
Ask first if you take thyroid medicine and want to start an iodine supplement

Questions people ask

How long does iodine stay in your body?

More than 90% of the iodine people eat leaves in urine, so most of a dose the thyroid does not need is soon gone. The thyroid keeps a store of 10 mg or more, and in a tracer study the body's iodine was predicted to take about 10 months to turn over completely.^1,5

When will I notice a difference?

If you already get enough iodine, there may be nothing to notice: the thyroid takes only what it needs. If intake was low, urinary iodine had more than doubled by 28 weeks in the trial in children, and trials measured thinking and development over months to years rather than days.^1,2

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Sources

*These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.

This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.