Iodine

What it is

The thyroid gland needs iodine to make thyroid hormones, which regulate growth, development and metabolism [1]. Most iodine in the diet comes from iodised salt, seafood and seaweed, dairy, and grains grown in iodine-rich soils [1]. Common supplement forms are potassium iodide and kelp (seaweed) powder, and trials have also used potassium iodate and single doses of iodised oil [2][3][4]. Some countries fortify salt by law, while others have no national iodisation policy, and mild deficiency has re-emerged in the UK, particularly among women of reproductive age and people who avoid dairy and fish [5]. Researchers describe mildly-to-moderately deficient pregnant women as those with a median urine iodine concentration of 50 to 149 µg/L [6]. Requirements rise in pregnancy and breastfeeding; the American Thyroid Association recommends a daily 150 µg iodine supplement for women in the US who are pregnant, lactating or planning pregnancy [2].

How it might work

Iodine is built into the thyroid hormones thyroxine (T4) and triiodothyronine (T3), which are needed for normal growth and brain development; in severe deficiency these hormones fall short, and endemic goitre and impaired development follow [1]. Iodine status is usually assessed from urine iodine concentration, and thyroglobulin is a blood marker that falls when iodine intake improves [3]. Too much iodine can also cause thyroid problems [7]. In animal models, extra iodine can trigger or worsen thyroid autoimmunity, but whether it acts directly on immune cells or through damage to thyroid tissue is not clear [8]. The thyroid antibodies used to diagnose autoimmune thyroid disease vary between populations under genetic and environmental influences, and their presence does not always mean disease will develop [8].

What the research says

Goitre and thyroid disease in deficiency: a 2014 meta-analysis of before-and-after studies from mainland China found goitre prevalence fell from 22.8% before universal salt iodisation to 12.6% afterwards, although the authors noted that goitre and nodules rose again after 2002, which they linked to possible excessive iodine intake [9]. A randomised trial in schoolchildren found abnormal thyroid volume fell from 18% at baseline to under 5% within 12 months when iodised salt (25 ppm) was supplied with or without an iodised-oil capsule, and to 9% at 18 months when families bought market salt [10]. In a randomised trial in Zaire, a single small dose of oral iodised oil (47 or 118 mg iodine) shrank visible goitres in most adults over 12 months [4]. The 2026 Cochrane review of 8 randomised trials in 2,528 children found iodine likely reduces hypothyroidism (risk ratio 0.03, 95% CI 0.00 to 0.23) and that a single 150 µg daily dose slightly improved cognition (0.19 standard deviations, 95% CI 0.04 to 0.34); the goitre result was very uncertain overall but fell sharply in a sensitivity analysis that removed a high-risk-of-bias trial (risk ratio 0.31, 95% CI 0.22 to 0.45) [1]. A 19-study meta-analysis in Ethiopia found goitre in about 40% of children and called for stronger salt iodisation [11].

Cognition in iodine-deficient children: a randomised trial in 184 mildly deficient New Zealand children found 28 weeks of 150 µg iodine a day raised the overall cognitive score by 0.19 standard deviations (P=0.011), with gains on two of four subtests [12]. In 310 moderately deficient Albanian children, a single 400 mg iodised-oil dose improved performance on four of seven cognitive and motor tests after 24 weeks [13]. However, a trial in 305 moderately deficient Bangladeshi children found no difference in cognitive or motor function four months after a 400 mg dose [14], and a 2014 review pooling two school-age trials found gains in perceptual reasoning (standardised mean difference 0.55, 95% CI 0.05 to 1.04) and a global cognitive index (0.27, 95% CI 0.10 to 0.44) with significant heterogeneity between studies [15].

Pregnancy: where deficiency is severe, iodised oil given before or during pregnancy prevents endemic cretinism and brain damage [16], and a systematic review of 8 trials found prenatal iodine reduced cretinism risk in severe deficiency but did not improve childhood intelligence, growth or pregnancy outcomes [17]. In mild-to-moderate deficiency the results are different: a trial in 832 pregnant women in India and Thailand found 200 µg a day had no effect on children's verbal IQ (difference -0.7, 95% CI -2.9 to 1.5), performance IQ (-1.6, -4.5 to 1.3) or executive function (-0.9, -6.8 to 5.0) at ages 5 to 6 [18]. A 2020 review of 37 studies found mostly no effect on maternal or infant thyroid hormones and no effect on child cognitive, language or motor scores in two pooled trials [6], and a 2021 meta-analysis of 14 trials found no evidence of improved growth or neurodevelopment [19]. Iodine does change iodine-status markers: pooled data from iodine-deficient pregnant women found lower TSH (standardised mean difference -0.14) and thyroglobulin (-0.22) with supplementation [20]. The 2017 Cochrane review of 14 studies found iodine decreased postpartum hyperthyroidism (risk ratio 0.32, 95% CI 0.11 to 0.91; low-quality evidence) while one small trial reported more digestive intolerance (risk ratio 15.33; very low-quality evidence) [21]. In moderate-to-severe deficiency without iodised salt, a single 400 mg iodised-oil dose to breastfeeding mothers kept infants' iodine status adequate for 6 months, while direct infant dosing was less effective [22]. Observational data link first-trimester deficiency (urine iodine/creatinine below 150 µg/g) to about 0.6 points lower verbal IQ, a difference that was not statistically significant [23].

People who already get enough iodine: in a trial of 100 older New Zealanders, 80 µg a day for 3 months did not change TSH, free T3 or free T4, although thyroglobulin fell 24% versus placebo [24]. In 112 mildly deficient adults, 150 µg a day for 24 weeks lowered thyroglobulin by 27% but did not change TSH or free T4 [3]. In a trial of 180 breastfeeding mothers in Iran, where salt iodisation is effective, 150 or 300 µg a day improved the mothers' own iodine status, but infants were iodine-sufficient in every group and the authors concluded supplements were unnecessary [25]. A 2023 review of 11 pregnancy studies similarly found 200 µg a day mainly changed urine iodine concentration, with some studies showing no thyroid hormone effect [26].

Autoimmune thyroid disease: the evidence is mixed. A 2023 systematic review in Graves' disease, covering 2 randomised trials and 3 observational studies, found adequate iodine intake was linked to lower recurrence and better control on antithyroid drugs than intake that was too low or too high, but the authors called the evidence suboptimal and could not pool the studies [27]. Reviews disagree on whether iodine programmes increase thyroid autoimmunity: one concludes that higher-than-adequate population intake from poorly controlled programmes can cause thyroid dysfunction including autoimmune thyroiditis, mostly subclinical [8], while another concludes that early post-iodisation rises in thyroid antibodies are largely transient and not clinically relevant [28]. In a Danish cohort of 1,278 pregnant women studied after national iodisation, thyroid dysfunction was found in 10% to 15.8% and was associated with antibody positivity, but not with preterm delivery (odds ratio 0.6, 95% CI 0.1 to 2.3) [29]. An older prospective study of 262 people found thyroid antibodies appeared in 7.5% of iodine-treated participants, mostly at low levels, with more reactions at 500 µg a day (14.8%) than at 200 µg (5%) and no hypothyroidism [30]. A small randomised postpartum study in 70 women found 11 mild, transient thyroid-function changes that were similar in the 50 µg and 250 µg groups [31]. In 20 pregnant women with Hashimoto's thyroiditis, antibody levels fell in 18 of 20 during 100 to 150 µg daily supplementation [32].

Excess iodine and kelp: a systematic review of 50 studies found that populations with excessive iodine intake had nearly three times the odds of overt hypothyroidism (odds ratio 2.78, 95% CI 1.47 to 5.27) and about double the odds of subclinical hypothyroidism (2.03, 1.58 to 2.62) compared with adequate-intake populations, with the pooled estimates coming from cross-sectional studies [33]. In a randomised trial where a supplement formulation error delivered more than 50 mg of iodate a day for 8 weeks, 10 of 43 older participants developed a raised TSH and 3 developed a suppressed TSH below 0.10 mIU/L; most values had normalised within 4 weeks of stopping [34]. In a small randomised trial in 36 healthy euthyroid adults, 4 weeks of kelp capsules raised TSH in both dose groups (P=0.04 and P=0.002) and lowered total T3 at the higher dose [35]. Seaweed delivering 475 µg iodine a day for 7 weeks raised TSH in 25 postmenopausal women, from 1.69 to 2.19 µU/mL [36]. In 49 habitual seaweed consumers in Norway, median estimated iodine intake was 658 µg a day, above the tolerable upper intake level, and TSH fell significantly after six weeks without seaweed [37]. Kelp products are hard to dose: only 10% of 224 UK seaweed products stated their iodine content, and 26 products could deliver a serving above the European upper level of 600 µg a day [38], while 23.5% of iodine-containing US prenatal vitamins used kelp, an iodine source known to be highly variable [2]. A 2026 UK review describes kelp-based supplements that deliver supraphysiological iodine doses and a growing minority of patients with iodine-induced thyroid dysfunction [5].

Doses used in studies

Daily oral doses in the trials cited here: 80 µg for 3 months in older New Zealanders [24]; 150 µg for 24 to 28 weeks in adults and children [3][12]; 200 µg a day through pregnancy [18][26]; 150 or 300 µg a day for 12 months while breastfeeding [25]; 300 µg of potassium iodide a day during pregnancy [39]; 50 or 250 µg a day for 8 months after birth [31]; and 100 to 150 µg a day in pregnant women with Hashimoto's thyroiditis [32]. Deficiency programmes and trials often used single large doses instead: iodised oil 47 to 118 mg in adults with goitre [4], a single 400 mg dose in schoolchildren [10][13], about 100 mg given directly to infants or 400 mg to their mothers [22], and single doses of 400 to 490 mg or 150 µg daily in the children's trials reviewed by Cochrane [1]. Iodised salt was tested at 25 ppm iodine in a school trial [10]. Kelp and seaweed studies supplied iodine through food: 475 µg a day for 7 weeks [36], about 1.03 mg a day for 8 weeks in a Japanese trial [40], and an accidental exposure above 50 mg a day for 8 weeks [34].

Safety and interactions

Common side effects: in the pregnancy Cochrane review, iodine supplements increased digestive intolerance about 15-fold in one small trial (risk ratio 15.33, 95% CI 2.07 to 113.70; very low-quality evidence) [21]. Thyroid test changes from high-dose iodate were mostly transient and resolved within 4 weeks after stopping [34].

Upper intake levels: the European tolerable upper intake level for adults is 600 µg a day [38], and Japan sets 2,200 µg a day [41]. Habitual seaweed consumers in Norway averaged 658 µg a day, above the European level [37].

Excess and thyroid harm: pooled observational data link excess iodine intake to about double the odds of subclinical hypothyroidism and nearly three times the odds of overt hypothyroidism compared with adequate intake [33]. Small randomised trials found kelp and seaweed raised TSH [35][36], and high-dose iodate produced both underactive and overactive thyroid test results in some older adults [34]. Kelp product labels rarely state iodine content, and iodine levels vary widely between products [38][2].

Thyroid medicines and Graves' disease: in patients taking antithyroid drugs for Graves' disease, adequate iodine intake was linked to better control and fewer relapses than intake that was too low or too high [27]. An older study found antibody reactions were more common at 500 µg a day than at 200 µg, and that iodine combined with thyroxine treatment showed fewer reactions [30]. The studies cited on this page report no interactions with other prescription medicines [21][27].

Pregnancy and breastfeeding: the American Thyroid Association recommends 150 µg a day for women in the US who are pregnant, lactating or planning pregnancy [2]. In severe deficiency, iodised oil prevents cretinism [16][17]; in mild-to-moderate deficiency, trials show no clear benefit for the child's development [18][6]. Excess iodine in pregnancy is considered a risk and is best avoided [42].

Thyroid conditions: in pregnant women with Hashimoto's thyroiditis, 100 to 150 µg a day did not worsen antibody levels in a small study [32]. People with thyroid disease, or taking thyroid medicines, should speak to a clinician before taking iodine supplements; this page is not medical advice [27][32].

Kidney and liver disease: the studies cited here report no results for people with kidney or liver disease [21][25].

Caveats and open questions

Goitre: the randomised evidence is old, and the 2026 Cochrane review rated its main goitre result very uncertain, with 97% heterogeneity between the two trials; the clear reduction appeared only after removing one high-risk-of-bias study [1]. The Chinese prevalence data come from before-and-after surveys and cannot prove cause and effect, and the same analysis linked very high iodine to more goitre after 2002 [9]. Modelling of China's programme suggests 15 to 18 mg/kg salt iodine is optimal for most villages, below current policy levels [7].

Cognition: the trials disagree - two found small gains [12][13] and one found none [14] - and the pooled cognitive estimate rests largely on a single trial [1].

Pregnancy: reviews conclude there is not enough good evidence to support routine iodine supplements in mild-to-moderate deficiency [6][42], and the only large modern trial found no neurodevelopmental benefit [18].

Autoimmunity: whether iodine programmes increase thyroid autoimmunity is still debated, and monitoring is advised so that population iodine intake stays below a median of 300 µg/L [8][28]. Not everyone responds to extra iodine the same way: in an 8-week Japanese trial, kelp providing about 1.03 mg iodine a day did not change thyroid hormone levels in overweight adults who already ate seaweed regularly [40].

Population monitoring: in Denmark, careful salt iodisation was followed by more diagnosed hypothyroidism in young and middle-aged adults [43][44], but a later survey found the overt hypothyroidism rate unchanged and overt thyrotoxicosis roughly halved [45]. Ongoing studies listed below are tracking iodine status, thyroid function and child development.