Iodine
Trace element the thyroid needs to make its hormones T4 and T3
Also known as
Definition
Iodine is an essential trace element that the body must obtain from food. The thyroid needs it to make the hormones thyroxine (T4) and triiodothyronine (T3). A healthy adult with adequate intake carries about 15–20 mg of iodine, 70–80% of it in the thyroid. More than 90% of the iodine taken in is excreted in the urine, which is why iodine status is usually measured in urine.
| Parameter | Value |
|---|---|
| Unit | µg/l (iodine concentration in urine) |
| Reference Range | There is no simple reference range for a single iodine measurement. The WHO assesses the iodine status of population groups by the median iodine concentration in urine: 100–199 µg/l is considered adequate in children and non-pregnant adults, 150–249 µg/l in pregnancy and above 100 µg/l during breastfeeding. These group values are not a target for you. Labs differ; the range on your own report is what counts, and your doctor interprets the result. |
Agree on your personal target range with your doctor.
µg/l corresponds to mcg/L on US lab reports; the numbers are identical.
↓ What a low value means
A low iodine concentration in urine can be related to low iodine intake. Iodine deficiency lasting many months can lead to an enlarged thyroid (goiter) or an underactive thyroid (hypothyroidism). Deficiency can occur where soils contain little iodine; pregnant women and people who do not use iodized salt are among those at higher risk. A single urine sample is not enough to establish a deficiency in one person. Discuss any deviations with your doctor.
↑ What a high value means
A high iodine concentration in urine can be related to high iodine intake, for example from food or supplements. Very high intake over a long period can have effects similar to deficiency: goiter, raised TSH and hypothyroidism; in some people it can also trigger an overactive thyroid. People with autoimmune thyroid disease may react to amounts considered safe for the general population. High amounts of iodine taken together with antithyroid medicines can promote hypothyroidism. Discuss unusual values with your doctor.
When to test
Measuring iodine in spot urine samples is mainly used to assess the iodine status of population groups. For an individual, a single spot sample is not considered a suitable measure; several 24-hour urine collections or several spot samples are more accurate. Whether a measurement makes sense in your situation is something to clarify with your doctor; mention any supplements you take.
Frequently asked questions
How much iodine does the body need? +
The US reference values (Recommended Dietary Allowance) are 150 µg per day for adolescents from age 14 and adults, 220 µg during pregnancy and 290 µg during breastfeeding. The tolerable upper intake level for adults there is 1,100 µg per day. Which amount suits your situation is something to discuss with your doctor.
Which foods contain iodine? +
The main source is iodized salt, meaning table salt with added iodine. Sea fish such as cod and seaweed such as kelp are naturally rich in iodine. Dairy products and plants grown in iodine-rich soil also provide iodine.
Why is iodine measured in urine rather than blood? +
The body excretes more than 90% of dietary iodine in the urine, so the urine value directly reflects intake. Single spot samples are suitable for statements about population groups; for an individual, several samples are more accurate.
How is iodine related to thyroid values? +
The pituitary gland controls the thyroid via TSH. Without enough iodine, TSH can stay elevated and the thyroid can enlarge. Very high iodine intake can also raise TSH. How iodine, TSH and free T4 fit together is for your doctor to interpret.
Iodine
Last updated: October 05, 2026
This information is for orientation only and does not replace medical advice. Reference ranges can vary by laboratory, method and country.