Why Iodine Is a Balancing Act
The thyroid uses iodine to make its hormones, so iodine status and thyroid health are tightly linked. Unusually, both too little and too much iodine cause similar problems. The NIH Office of Dietary Supplements notes that 'high intakes of iodine can cause some of the same symptoms as iodine deficiency—including goiter, elevated TSH levels, and hypothyroidism,' and that 'iodine-induced hyperthyroidism can also result from high iodine intakes' [1].
How Much Adults Need
The RDA is 150 mcg/day for adults, rising to 220 mcg in pregnancy and 290 mcg during lactation [1]. The Tolerable Upper Intake Level is 1,100 mcg/day [1].
Pregnancy Is Especially Important
Iodine needs rise in pregnancy because the developing fetus depends on the mother's supply. Per NIH, in early pregnancy 'the fetus depends entirely on maternal T4 and, therefore, on maternal iodine intake,' and thyroid hormone production 'increases by approximately 50% during pregnancy' [1]. Many prenatal vitamins include iodine for this reason.
Where Over-Intake Comes From
Most people get enough iodine from iodized salt, dairy, seafood, and eggs. The common route to *excess* is high-dose kelp or seaweed supplements, which can contain enormous and variable amounts of iodine — sometimes far above the upper limit in a single serving. 'Thyroid support' supplements with added iodine can also push intake too high.
Practical Guidance
- Aim for adequacy (iodized salt and a normal diet usually suffice); pregnancy needs are higher.
- Be cautious with kelp/seaweed supplements and high-iodine 'thyroid' products — more is not better.
- If you have a thyroid condition, talk to a clinician before taking iodine, since both too little and too much can worsen thyroid problems.