The mineral nobody talks about.

Urinary iodine is how iodine status is measured across a population. In national survey data for US women of childbearing age, the median dropped from 135 to 103.5 µg/L between 2001 and 2020, and the pregnant and nursing subgroups sat below adequate the entire time.¹

That matters because iodine is the raw material of thyroid hormone, and thyroid hormone sets the pace for every cell you own. Deficiency and excess both show up as goiter, an underactive thyroid, thyroid autoimmunity and, in children, slower brain development.²

Iodine is a halogen. So are fluorine, chlorine and bromine, and perchlorate is an industrial salt that acts like one. Iodine reaches the thyroid through one door, the sodium-iodide symporter. In a 2026 modeling study, fluoride, chloride, bromide and perchlorate all bound that door, and the enzyme that builds the hormone, more tightly than iodide did.³ The symporter's own preference order puts perchlorate ahead of iodide.⁴

Perchlorate competitively blocks the symporter, lowering thyroid hormone output and potentially raising TSH, most of all in pregnant and iodine-deficient people.⁵ In more than 20,000 US adults, higher urinary perchlorate was linked to self-reported thyroid and pancreatic cancer, without a clean dose-response.⁶ Fluoride: five of seven studies in a systematic review tied high exposure to altered thyroid function, with the overall evidence rated low quality,⁷ and in the Canadian MIREC pregnancy cohort it tracked with higher maternal TSH.⁸ Bromide lowers thyroid iodide uptake and raises the iodide lost in urine.⁹ Rats fed brominated vegetable oil developed higher TSH and lower T4,¹⁰ and an Ethiopian survey found potassium bromate in every bread sample tested, above the WHO/FDA limit.¹¹

Pregnancy raises the requirement three ways: the kidneys clear more, the thyroid makes more, and iodine crosses to the baby.¹² Deficiency impairs fetal brain development and raises the risk of growth restriction, and the estrogen stimulation of assisted reproduction shifts the thyroid's demand again.¹³

The children carry it. In a 15-year follow-up of 266 children, those whose mothers ran below 150 µg/L in pregnancy had lower spelling scores from Year 3 through Year 9, despite adequate iodine in childhood.¹⁴ In a UK birth cohort, severe first-trimester deficiency went with lower vocabulary and IQ at age 15.¹⁵ And for the mother: a 2026 scoping review found low iodine intake consistently tied to postpartum depressive symptoms and anhedonia, while supplementation's effect on mood was inconsistent.¹⁶

Selenium is the pair. The enzymes that turn T4 into active T3 are selenium enzymes; without selenium, conversion stalls and T4 piles up.²³ In midlife New Zealand women, iodine deficiency and low selenium were common together, and plasma selenium predicted the T3-to-T4 ratio.²⁴ Selenium can lower TPO antibodies in Hashimoto's.²⁵ The other side: the GRASS trial found no gain in remission or quality of life in newly diagnosed Graves' disease,²⁶ antibodies unexpectedly rose in one Hashimoto's group after six months,²⁷ and in 2,347 matched pairs who were not deficient, supplementing went with higher TSH, more antibodies and higher all-cause mortality.²⁸ Selenium is a dose. Test it like iodine (selenium in the Lab Library).

Magnesium. In 104 patients, low serum magnesium came with higher TPO and thyroglobulin antibodies and more Hashimoto's-pattern ultrasounds.²⁹ Low magnesium and low vitamin D were each tied to goiter, hypothyroidism and antibody positivity.³⁰ Potassium. Fewer than 2% of US adults met the potassium recommendation, while 99.4% ate too much sodium.³¹ A normal blood potassium can hide low potassium inside cells.³² Read magnesium and potassium in the Lab Library.

Vitamin C. In Hashimoto's patients in national survey data, the highest vitamin C intake went with half the odds of hypothyroidism.³³ In a pilot trial, 1 g a day showed potential benefit on symptoms and TSH in people on high-dose levothyroxine.³⁴ Vitamin E. In a 2026 pilot study of 78 women with euthyroid Hashimoto's, selenomethionine improved mood and sexual function only in those with adequate vitamin E intake, not low and not high.³⁵ In a second pilot, on vitamin D, the largest gains also came with adequate vitamin E.³⁶