Thyroid Disorders in Women

References & Evidence Notes

Each numbered entry below is either a source you can follow or a note setting out what the evidence does and does not support. Both are numbered together so the markers in the text line up.

Last reviewed October 20, 2026. Supplement entries are cross-checked against the NIH National Center for Complementary and Integrative Health and the Linus Pauling Institute Micronutrient Information Center.⁸ Two things to hold on to. First, thyroid hormone replacement is not optional and should not be stopped or reduced without your clinician. Untreated hypothyroidism in pregnancy affects fetal brain development, and untreated hyperthyroidism causes atrial fibrillation, bone loss and, rarely, thyroid storm. Nothing on this page replaces levothyroxine. Second, the biggest nutritional trap in autoimmune thyroid disease runs the opposite way to intuition: more iodine is not better. Excess iodine, including from kelp and high-dose supplements, can trigger or worsen Hashimoto's and precipitate thyroid dysfunction. Correct a deficiency if you have one; do not supplement iodine speculatively. If you are pregnant or planning pregnancy, thyroid status should be checked and managed actively.

1. On the conditions: Hashimoto's thyroiditis is the most common cause of hypothyroidism in iodine-sufficient countries, and iodine deficiency remains the leading cause globally. Graves' disease is the most common cause of hyperthyroidism. Subclinical disease, meaning an abnormal TSH with normal free hormones, is common and is managed by risk rather than reflexively. ↩
2. On testing: TSH is the primary screening test, with free T4 added for confirmation and TPO antibodies to establish autoimmunity. Reverse T3 is not recommended for routine clinical use and is not a validated marker of tissue thyroid status; it rises in illness, fasting and stress as a normal adaptive response. Interpreting it as a diagnosis leads to treatment that is not indicated. ↩
3. On iodine, the point most likely to cause harm: the relationship between iodine and thyroid disease is U-shaped: both deficiency and excess cause dysfunction, a dose-response curve described in reviews of trace elements in autoimmune thyroid disease, PubMed 41383597. In populations with adequate iodine, increased intake is associated with higher rates of autoimmune thyroiditis, and high-dose supplements or kelp can precipitate hypothyroidism or hyperthyroidism. The adult tolerable upper intake level is 1,100 mcg per day, and supplements sold for thyroid support routinely exceed it by a wide margin. Pregnancy raises requirements, and prenatal supplements typically contain around 150 mcg, which is the appropriate context for supplementation. ↩
4. On selenium: selenium supplementation reduces TPO antibody levels in several randomized trials, though whether this changes clinical outcomes is less clear, and the CATALYST trial of selenium in Hashimoto's did not show benefit on thyroid function or quality of life. Selenium has a narrow therapeutic window; the upper intake level is 400 mcg per day and chronic excess causes selenosis. Two Brazil nuts a day is a meaningful dose. ↩
5. On myo-inositol with selenium: several small trials, largely from one research group in Italy, report improved TSH and reduced antibodies in subclinical Hashimoto's compared with selenium alone. The trials are small and have not been independently replicated at scale. Reasonable to try, and not established. ↩
6. On fluoride, goitrogens and other avoidance advice: the evidence linking fluoride to thyroid dysfunction comes largely from areas with naturally very high fluoride concentrations, well above fluoridated water supplies, and there is no good evidence that avoiding fluoridated water or toothpaste improves thyroid antibodies or function. Similarly, goitrogens in cruciferous vegetables and soy are not a practical problem at normal dietary intakes in iodine-sufficient people; soy can interfere with levothyroxine absorption, which is a timing issue rather than a reason to avoid it. Take it on an empty stomach 30 to 60 minutes before breakfast or coffee, keep calcium and iron supplements at least 4 hours away from it, and take it consistently. ↩
7. On desiccated thyroid extract and combination T4/T3: randomized trials have generally not shown superiority of combination therapy over levothyroxine alone on average, though a subgroup of patients reports preferring it, and this remains genuinely debated. Desiccated products have less consistent hormone content between batches. This is a discussion to have with an endocrinologist rather than a self-directed switch. ↩
8. National Center for Complementary and Integrative Health (NIH), nccih.nih.gov, and the Linus Pauling Institute Micronutrient Information Center, lpi.oregonstate.edu/mic. ↩
9. On gluten-free diets in Hashimoto's without celiac disease: a 2025 meta-analysis could pool only three randomized trials with 110 participants. The diet lowered thyroglobulin antibodies, raised TPO antibodies, and did not significantly change TSH, free T4 or free T3, and the evidence was rated very uncertain, PubMed 41228508. ↩
10. On how common thyroid disease is: the American Thyroid Association estimates that one woman in eight will develop a thyroid disorder during her lifetime, that women are five to eight times more likely than men to have thyroid problems, that up to 60% of people with thyroid disease are unaware of it, and that most thyroid cancers respond to treatment, although a small percentage can be very aggressive, American Thyroid Association. ↩
11. On thyroid nodules: by age 60 about half of all people have a nodule that can be found on examination or imaging, the cause of most is unknown, over 90% are benign, and benign nodules are watched with ultrasound every 6 to 12 months, American Thyroid Association, thyroid nodules. ↩
12. On postpartum thyroiditis: the American Thyroid Association reports that it occurs in about 5 to 10% of women in the United States, more often in women with thyroid antibodies, type 1 diabetes or a previous episode, and that about 20% of women have it again with a later pregnancy. Most regain normal thyroid function within 12 to 18 months of the first symptoms, but about 20% of those who go through an underactive phase stay underactive. If thyroid hormone is started, it is usually continued for about 6 to 12 months and then reduced to see whether it is still needed, American Thyroid Association, postpartum thyroiditis. ↩
13. On an underactive thyroid: NHS guidance names Hashimoto's disease as the main cause in the UK, most common in women and usually starting between 30 and 50, with thyroiditis, neck surgery or radiation therapy, treatment for an overactive thyroid, too little iodine and certain medicines as other causes, and a family history raising the risk. Low mood is among the common symptoms. Treatment is usually daily hormone replacement, often for life, with regular blood tests to check the dose, and the dose usually changes in pregnancy; with the right treatment most people live a normal life, NHS, underactive thyroid. ↩
14. On biotin and thyroid tests: high-dose biotin can interfere with the laboratory immunoassays used to measure thyroid hormones and TSH, giving falsely high or falsely low results that can look like an overactive thyroid, while ordinary dietary biotin does not typically reach levels that interfere, PubMed 31813616, PubMed 31473202. ↩
15. On taking thyroid hormone replacement: NHS guidance advises taking it on an empty stomach 30 to 60 minutes before the first meal of the day, any drink with caffeine such as tea or coffee, or any other medicine, and lists calcium and iron supplements, antacids and biotin among the things that can interfere with it or with thyroid tests. MedlinePlus advises taking calcium carbonate or iron supplements at least 4 hours before or after it, notes that foods containing soybeans, walnuts and dietary fiber may affect how it works, that it may take several weeks to notice a change, and advises not stopping it without talking to your doctor, NHS medicines guide, MedlinePlus drug information. ↩
16. On an overactive thyroid: NHS guidance states that about 4 in every 5 cases are caused by Graves' disease, an autoimmune condition that mostly affects young or middle-aged women, often runs in families and is more likely in smokers, and that nodules, iodine-containing medicines and high pregnancy hormone levels early in pregnancy are other causes. Antithyroid medicine is usually taken for 12 to 18 months, with benefit taking a few months, and some people need it for years; a beta blocker may ease symptoms meanwhile; a high temperature, sore throat or persistent cough while taking it needs a doctor at once. Radioactive iodine often leaves hormone levels too low, and after the whole thyroid is removed, hormone replacement is needed for life, NHS, overactive thyroid. ↩
17. On iodine: MedlinePlus notes that many months of too little iodine can cause goiter or an underactive thyroid and that very high iodine intake can reduce thyroid function. NHS guidance adds that high doses taken for long periods could change the way the thyroid works, and that a raised iodine level, for example from an iodine-containing medicine, can make the thyroid overactive, MedlinePlus, iodine in diet, NHS, iodine. ↩
18. On ashwagandha: NCCIH states that it is not recommended for people who have autoimmune or thyroid disorders, that it may interact with thyroid hormone medicines, that it should be avoided during pregnancy and not used while breastfeeding, and that it has rarely been linked to liver injury, NCCIH, ashwagandha. ↩
19. On iron-rich foods: MedlinePlus lists dried beans, dried fruits, eggs, iron-fortified cereals, liver, lean red meat, oysters, poultry, salmon, tuna and whole grains among the best sources of iron, and among plant sources beans, peas, lentils, soybeans, broccoli, spinach, kale and collards. Eating some lean meat, fish or poultry with beans or dark leafy greens can improve absorption of the plant iron up to three times, and foods rich in vitamin C such as citrus, strawberries, tomatoes and potatoes also increase absorption, MedlinePlus, iron in diet. ↩
20. On liver and vitamin A: NHS guidance advises not eating liver or liver products more than once a week and not taking supplements containing vitamin A (including fish liver oil) in weeks you eat liver. It advises avoiding liver altogether in pregnancy, because its high vitamin A content can harm a baby, NHS, vitamin A, NHS, foods to avoid in pregnancy. ↩
21. On vitamin D in autoimmune thyroiditis: a 2018 meta-analysis of 6 randomized trials with 344 patients found that vitamin D lowered TPO antibodies at about six months but not at three months or less, and lowered thyroglobulin antibodies, with no serious adverse effects; the authors call for better trials, especially of long-term effects, PubMed 29388046. ↩
22. On a lump in the neck: NHS guidance advises seeing a doctor for a new lump in the neck or an existing one that is getting bigger, or a hoarse voice, sore throat or cough lasting more than 3 weeks, and asking for an urgent appointment for difficulty swallowing or breathing; these symptoms have many causes and do not definitely mean cancer, NHS, thyroid cancer symptoms. ↩
23. On thyroid storm: MedlinePlus lists agitation, a change in alertness, confusion, diarrhea, a raised temperature, a pounding heart, restlessness, shaking and sweating as symptoms of this life-threatening flare of an overactive thyroid and advises calling emergency services; NHS guidance lists a rapid heartbeat, a high temperature, yellowing of the skin and eyes, severe agitation and confusion and loss of consciousness, and notes that infection, pregnancy or not taking medicine correctly can trigger it, MedlinePlus, thyroid storm, NHS, overactive thyroid complications. ↩
24. On myxedema coma: MedlinePlus describes it as the rare, most severe form of an underactive thyroid and a medical emergency that must be treated in the hospital, with a below-normal body temperature, slowed breathing, low blood pressure, low blood sugar and unresponsiveness, often set off by infection, illness, cold, certain medicines or not taking thyroid hormone consistently; the MSD Manual notes that an untreated underactive thyroid can progress to confusion, stupor or coma, MedlinePlus, hypothyroidism, MSD Manual, hypothyroidism. ↩
25. Gencer B, Djousse L, Al-Ramady OT, Cook NR, Manson JE, Albert CM. Effect of long-term marine omega-⁠3 fatty acids supplementation on the risk of atrial fibrillation in randomized controlled trials of cardiovascular outcomes: a systematic review and meta-analysis. Circulation. 2021;144(25):1981-⁠1990. PubMed 34612056. Seven trials with 81,210 people followed for about 5 years: omega-⁠3 supplements were linked to a higher risk of atrial fibrillation (hazard ratio 1.25), more so in trials of more than 1 g a day (1.49), and the risk rose with each additional gram. ↩