Hypothyroidism happens when your thyroid gland can’t produce enough hormones to meet your body’s needs. Nearly 5 out of 100 Americans ages 12 and older have the condition, though most cases are mild. The causes range from immune system attacks on the thyroid to iodine deficiency, medical treatments, medications, and conditions you’re born with.
Autoimmune Disease Is the Leading Cause
In countries with adequate iodine intake, an autoimmune condition called Hashimoto’s thyroiditis is the most common cause of hypothyroidism. Your immune system mistakenly identifies thyroid tissue as a threat and mounts a sustained attack against it. Certain white blood cells infiltrate the thyroid, causing inflammation and destroying the cells that produce hormones. Over time, enough tissue is damaged that the gland can no longer keep up with demand.
The attack involves specific antibodies that target thyroid components. The most common, found in over 90% of people with Hashimoto’s, targets an enzyme the thyroid needs to make its hormones. Another antibody, present in 50% to 80% of cases, attacks a protein the gland uses to store those hormones. A third type blocks the receptor that normally tells the thyroid to ramp up production. The combined effect of these antibodies and the direct destruction by immune cells leads to progressive scarring and shrinkage of the gland.
Women are significantly more likely to develop Hashimoto’s than men. The condition can appear at any age, but the peak number of cases occurs between ages 36 and 45. Having one autoimmune disease raises your risk of developing others, so people with type 1 diabetes, celiac disease, or rheumatoid arthritis are more likely to also develop autoimmune thyroid disease.
Iodine Deficiency
Your thyroid needs iodine to build its hormones. Without enough of it, the gland simply cannot manufacture what the body requires. When iodine intake drops below about 100 micrograms per day, your brain responds by sending stronger signals to the thyroid, essentially telling it to work harder. This sustained overstimulation causes the gland to enlarge, a condition known as goiter. If iodine levels remain too low, even that compensatory effort fails, and hormone production drops.
Adults need about 150 micrograms of iodine daily. Pregnant women need considerably more, around 220 to 250 micrograms, because they’re supplying iodine for fetal brain development as well. Iodine deficiency remains the leading cause of hypothyroidism worldwide, particularly in regions without iodized salt programs. In the United States and other countries where salt is routinely iodized, deficiency is uncommon but not impossible, especially among people who avoid processed foods and iodized salt.
Medical Treatments That Damage the Thyroid
Treatments aimed at an overactive thyroid often end up causing an underactive one. Radioactive iodine therapy, commonly used for Graves’ disease, works by destroying thyroid tissue to bring hormone levels down. But controlling the extent of that destruction is difficult. In one study of 272 patients treated for Graves’ disease, about 51% developed permanent hypothyroidism after one or more rounds of radioactive iodine. Most of those cases, roughly 74%, occurred after the first treatment.
Surgical removal of part or all of the thyroid (for cancer, large goiters, or overactive nodules) has a similar outcome. Removing the entire gland guarantees lifelong hypothyroidism, and even partial removal can leave too little tissue to produce adequate hormones.
Radiation therapy directed at the head or neck for cancers unrelated to the thyroid can also cause collateral damage. When radiation doses exceed a certain threshold, the pituitary gland or the thyroid itself can be harmed. One review found that external radiation for brain tumors caused central hypothyroidism in 65% of treated patients.
Medications That Interfere With Thyroid Function
Several widely prescribed drugs can suppress thyroid hormone production as a side effect. Lithium, used for bipolar disorder, is one of the best-known culprits. It concentrates in the thyroid and directly inhibits hormone release.
Amiodarone, a heart rhythm medication, poses a unique problem because it contains a large amount of iodine. That iodine overload triggers a protective mechanism in the thyroid called the Wolff-Chaikoff effect, where the gland temporarily shuts down hormone production to avoid making too much. In a healthy thyroid, this effect resolves on its own. In some people, particularly those with underlying autoimmune thyroid disease, the gland never fully recovers, and hypothyroidism becomes persistent.
Newer cancer immunotherapy drugs (checkpoint inhibitors) can also trigger thyroid inflammation by unleashing immune activity against thyroid tissue. This is an increasingly recognized cause as these treatments become more common.
Postpartum Thyroiditis
Some women develop thyroid inflammation in the months after giving birth. The condition typically unfolds in two phases. First, between 1 and 4 months postpartum, stored hormones leak from the inflamed gland, causing a temporary period of excess thyroid hormone. Then, around 4 to 8 months postpartum, the damaged gland can’t keep up, and hypothyroidism sets in. Symptoms during this phase include fatigue, cold sensitivity, weight gain, and depression, which can easily be mistaken for normal postpartum adjustment.
Most women recover normal thyroid function within 3 to 6 months after the hypothyroid phase begins. But up to 20% of those affected, particularly women who carry thyroid antibodies or have a history of autoimmune thyroid disease, develop permanent hypothyroidism and need lifelong hormone replacement.
Congenital Hypothyroidism
Some babies are born with a thyroid that didn’t form properly, formed in the wrong location, or doesn’t function. This is called congenital hypothyroidism, and it affects roughly 1 in every 2,000 to 4,000 newborns. Because thyroid hormones are critical for brain development, catching it early is essential. Standard newborn screening involves a heel-prick blood test performed between 48 and 72 hours after birth to measure thyroid hormone and stimulating hormone levels. Early treatment with thyroid hormone replacement prevents intellectual disability and developmental delays.
Pituitary and Brain-Related Causes
In a small percentage of cases, the thyroid itself is perfectly healthy, but the gland that controls it is not. The pituitary gland, a pea-sized structure at the base of the brain, produces the signaling hormone that tells the thyroid how much hormone to make. When the pituitary is damaged, that signal weakens or disappears, and thyroid hormone production drops even though the thyroid could technically still function.
The most common culprits in adults are pituitary tumors (particularly larger ones called macroadenomas), pituitary surgery, and radiation therapy aimed at the brain. Less common causes include Sheehan syndrome, a condition where severe blood loss during childbirth cuts off blood supply to the pituitary, and inflammatory or infiltrative diseases like sarcoidosis. In children, brain tumors called craniopharyngiomas and prior cranial radiation for childhood cancers are the usual triggers.
Certain Foods in Extreme Amounts
Cruciferous vegetables like cabbage, broccoli, cauliflower, kale, and turnips contain compounds that can interfere with thyroid hormone production. These compounds block the same enzyme that autoimmune antibodies target in Hashimoto’s. However, the real-world risk is minimal for most people. Cooking destroys the enzyme responsible for activating these compounds, largely eliminating their effect on the thyroid. You would need to eat very large quantities of raw cruciferous vegetables, consistently, for them to meaningfully affect your thyroid function.
Soy products have also drawn scrutiny. A review of 14 clinical trials concluded that soy protein does not affect normal thyroid function in people with sufficient iodine intake. Where soy becomes relevant is in people already taking thyroid hormone medication: it can reduce absorption of the synthetic hormone, potentially requiring a dose adjustment. Isolated case reports exist, including one of an infant fed exclusively soy formula from birth who developed hypothyroidism that reversed when the formula was stopped. But for adults eating a varied diet with adequate iodine, moderate soy consumption is not a meaningful thyroid risk.
Who Is Most at Risk
Women are far more likely to develop hypothyroidism than men, driven largely by higher rates of autoimmune thyroid disease. Estrogen appears to play a role in this disparity. Among women of childbearing age, 6% to 8% have mildly elevated thyroid-stimulating hormone levels indicating early or subclinical hypothyroidism, while 2% to 3% have clinical hypothyroidism requiring treatment.
Risk increases with age for both sexes, though the pattern differs. Younger women are more likely to have the mild, subclinical form, while the clinical and more severe forms become more common in middle age. A family history of thyroid disease or other autoimmune conditions is one of the strongest predictors. Having one affected first-degree relative significantly raises your own likelihood of eventually developing the condition.