Iodine and the Thyroid: Underactive, Overactive Thyroid and the Effects of Too Much Iodine
How too little and too much iodine can each affect the thyroid, what health agencies say about goiter, underactive and overactive thyroid, and why thyroid symptoms need a clinician.
Content last updated . This page has not been medically reviewed.
Please read first. Thyroid symptoms such as tiredness, weight change, a racing heart or a swollen neck have many possible causes, and they need blood tests and a clinician, not a supplement. The NHS says a swollen thyroid (goitre) is not usually serious but should be checked, and that trouble breathing or a wheezing noise when breathing needs urgent help. [1] In the US, call 911 for breathing trouble. This page is information only. It does not tell anyone to start, stop or change a medicine.
What the terms mean
| Term | How the sources describe it |
|---|---|
| Thyroid gland | A small, butterfly-shaped gland in the front of the neck that makes hormones controlling the way the body uses energy. [5] |
| Hypothyroidism (underactive thyroid) | The thyroid does not make enough thyroid hormones to meet the body's needs. [5] |
| Hyperthyroidism (overactive thyroid) | The thyroid makes more thyroid hormones than the body needs. [6] |
| Goiter (goitre) | An enlarged thyroid that may make the neck look swollen. [6] |
Too little iodine
MedlinePlus says many months of iodine deficiency may cause goiter or hypothyroidism, and that deficiency occurs where soil is iodine-poor. [2] It also says iodine deficiency is the most common cause of a goiter, and that using iodized table salt prevents most simple goiters. [7] The NHS says that in the UK an underactive thyroid is mainly caused by an autoimmune condition, and that not getting enough iodine is a rare cause there. [8] A 2025 JAMA review reports that the worldwide prevalence of hypothyroidism ranges from 0.3% to 12% depending on iodine intake. [9]Too much iodine
MedlinePlus lists too much iodine among the causes of hyperthyroidism and names some medicines, cough syrups, seaweed and seaweed-based supplements as iodine sources. [6] NIDDK (part of the US National Institutes of Health) says that in some people large amounts of iodine may cause the thyroid to make too much hormone. [10] A 2019 review says high iodine intakes are well tolerated by most healthy people, but in some they may precipitate hyperthyroidism, hypothyroidism, goiter or thyroid autoimmunity, and iodine-induced hyperthyroidism can be life-threatening in some individuals. [3] A 2024 review adds that iodine-containing medicines such as amiodarone and iodinated contrast media (used in some scans) are also sources of excess iodine. [11]What was found, by question
Evidence labels used on this page: established role, susceptible groups, mixed, Proper evidence not found.| Question | What the source found | Strength |
|---|---|---|
| Does low iodine cause goiter and hypothyroidism? | Yes, in places with iodine-poor soil; iodized salt prevents most simple goiters. [2][7] | Established role |
| Can excess iodine cause thyroid disease? | Usually mild and transient, but it can occur, and people with earlier thyroid disease or earlier iodine deficiency may be more susceptible. [3] | Susceptible groups |
| Is kelp or seaweed a concern? | NIDDK says foods with large amounts of iodine, such as kelp, dulse or other seaweed, may cause or worsen hypothyroidism or hyperthyroidism in people with autoimmune thyroid disorders. [4][10] | Susceptible groups |
| Do iodine supplements treat a thyroid condition in people with enough iodine? | Proper evidence not found. We read NHS, MedlinePlus and NIDDK thyroid pages and searched PubMed for trials of iodine in adults with hypothyroidism; none described iodine as a treatment. | Not supported by sources |
| Does cutting iodine help Hashimoto's disease? | A 2023 systematic review of nutrition studies in Hashimoto's thyroiditis found one study of dietary iodine restriction among nine included, so this question is unsettled. [12] | Mixed |
The last row concerns Hashimoto's disease (an autoimmune cause of underactive thyroid). It is a reason to ask a clinician, not a reason to change iodine intake alone.
Official intake levels
| Body and year | Group | Level |
|---|---|---|
| US Dietary Reference Intakes, via MedlinePlus (reviewed 2025) | Age 14 and older; pregnancy; breastfeeding | RDA 150; 220; 290 micrograms (mcg) of iodine a day [2] |
| NHS (UK), page reviewed 2020 | Adults | 140 mcg a day; supplements of 0.5 mg (500 mcg) or less a day are unlikely to cause harm [13] |
| EFSA (EU), 2025 overview | Adults, including pregnancy and breastfeeding | Tolerable upper intake level 600 mcg a day from all dietary sources [14] |
| Health Canada dietary reference intake tables | Adults; pregnancy and breastfeeding | Upper level 1,100 mcg a day [15] |
Bodies differ, none of these is a target, and an upper level is not a safe amount for everyone, especially people with thyroid disease. See Iodine for the full table and Foods high in iodine.
Tests and treatment are clinician-led
The NHS says an underactive thyroid is diagnosed with a clinician's questions, a neck examination and blood tests for thyroid-stimulating hormone (TSH) and thyroxine (T4), and is usually treated with levothyroxine tablets. [8] MedlinePlus lists a goiter work-up that can include thyroid blood tests, a thyroid scan and an ultrasound. [7] Radioactive iodine used as a hyperthyroidism treatment is a medical therapy, not a supplement, and NIDDK says it is not used for women who are pregnant or breastfeeding. [10]Frequently asked questions
Can too much iodine cause an underactive thyroid?
Yes, in some people. MedlinePlus says very high intake of iodine can reduce the function of the thyroid gland. [2] A clinician can check thyroid blood tests if there is concern.
Can too much iodine cause an overactive thyroid?
It can in some people. MedlinePlus lists too much iodine among the causes of hyperthyroidism, and NIDDK says large amounts of iodine may cause the thyroid to make too much hormone in some people. [6][10]
Should I take kelp if my thyroid is underactive?
The sources we read do not recommend it. NIDDK says kelp and other seaweed may cause or worsen hypothyroidism in people with autoimmune thyroid disorders, and to talk with the health care team about iodine supplements. [4]
Does iodine deficiency still happen?
Yes. A 2009 review estimated about 2 billion people worldwide have insufficient iodine intake, and the NHS calls it rare in the UK. [17][8]
Can an iodine blood or urine test tell me if I need iodine?
Not on its own. Urinary iodine is used to judge populations, and a 2009 review says assessing iodine status in pregnancy is difficult. [3][17] Thyroid blood tests are ordered by a clinician.
More details: evidence, limits and cautions
The details behind this page: cautions, limits of the evidence and where sources disagree. Open any note to read it.
Cautions
- Medicines. MedlinePlus says taking high doses of iodine with anti-thyroid medicines can have an additive effect and could cause hypothyroidism. [2] A review of levothyroxine interactions noted that chromium supplements, among others, may change how levothyroxine works and need more study. [16] Ask a pharmacist before adding any supplement; see medication interactions.
- Pregnancy and breastfeeding. Needs are higher and thyroid disease matters for both parent and baby; see iodine in pregnancy and breastfeeding.
- Children. The sources we read for this page do not give iodine supplement advice for children; ask a pediatrician.
- Kidney and liver conditions, and before surgery. The sources we read do not address iodine supplements in these situations, which is not proof of safety. Tell the team about every supplement; see supplements before surgery.
- Quality. Iodine content of seaweed products varies enormously; see kelp and seaweed supplements and quality testing.
For the wider picture of tiredness and thyroid problems see Thyroid problems and fatigue.
References
- NHS (UK). Goitre (page last reviewed 11 March 2026). Link. Read 2026-10-23.
- MedlinePlus (US National Library of Medicine). Iodine in diet (review date 21 January 2025). Link. Read 2026-10-23.
- Excess iodine intake: sources, assessment, and effects on thyroid function. Ann N Y Acad Sci. 2019. Link. Read 2026-10-23.
- National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK, US). Hypothyroidism (underactive thyroid) (last reviewed March 2021). Link. Read 2026-10-23.
- MedlinePlus (US National Library of Medicine). Hypothyroidism (health topic summary). Link. Read 2026-10-23.
- MedlinePlus (US National Library of Medicine). Hyperthyroidism (health topic summary). Link. Read 2026-10-23.
- MedlinePlus (US National Library of Medicine). Simple goiter (review date 25 January 2026). Link. Read 2026-10-23.
- NHS (UK). Underactive thyroid (hypothyroidism) (page last reviewed 28 April 2025). Link. Read 2026-10-23.
- Hypothyroidism: A Review. JAMA. 2025. Link. Read 2026-10-23.
- National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK, US). Hyperthyroidism (overactive thyroid) (last reviewed August 2021). Link. Read 2026-10-23.
- Risks of Iodine Excess. Endocr Rev. 2024. Link. Read 2026-10-23.
- The Influence of Nutritional Intervention in the Treatment of Hashimoto's Thyroiditis: A Systematic Review. Nutrients. 2023. Link. Read 2026-10-23.
- NHS (UK). Vitamins and minerals: Iodine (page last reviewed 3 August 2020). Link. Read 2026-10-23.
- European Food Safety Authority. Overview on Tolerable Upper Intake Levels as derived by the SCF and the EFSA NDA Panel, version 11 (August 2025). Link. Read 2026-10-23.
- Health Canada. Dietary reference intakes: reference values for elements (tables). Link. Read 2026-10-23.
- Wiesner A et al. Levothyroxine Interactions with Food and Dietary Supplements: A Systematic Review. Pharmaceuticals (Basel). 2021. Link. Read 2026-10-23.
- Zimmermann MB. Iodine deficiency. Endocr Rev. 2009. Link. Read 2026-10-23.