Hypothyroidism has many possible causes, but in countries with adequate iodine intake, the most common one is an autoimmune condition called Hashimoto’s thyroiditis. Other causes range from medical treatments and medications to iodine imbalances, pituitary gland problems, and conditions present at birth. Global prevalence falls between 0.3% and 5.3%, with women affected far more often than men.
Hashimoto’s Thyroiditis
Hashimoto’s is the leading cause of hypothyroidism in the United States and throughout the developed world. It’s an autoimmune disorder, meaning the body’s own immune system mistakenly targets the thyroid gland. Specifically, certain immune cells infiltrate the thyroid and destroy the cells responsible for making thyroid hormones. Over time, this ongoing attack causes inflammation and scarring that progressively erodes the gland’s ability to function.
The process involves two arms of the immune system working together. One type of white blood cell directly kills thyroid tissue, while another activates the production of antibodies that target proteins the thyroid needs to manufacture hormones. This double assault is why Hashimoto’s tends to cause a slow, steady decline in thyroid function rather than a sudden crash. Many people have the condition for years before their hormone levels drop low enough to cause noticeable symptoms.
Radioactive Iodine and Thyroid Surgery
Some of the most common non-autoimmune causes of hypothyroidism are, paradoxically, treatments for other thyroid problems. People with an overactive thyroid (Graves’ disease) or thyroid cancer often receive radioactive iodine therapy or have part or all of their thyroid gland surgically removed. Both treatments can leave the gland unable to produce enough hormones.
Radioactive iodine is particularly likely to cause hypothyroidism over time. In one study of patients treated for Graves’ disease, about 37.5% developed hypothyroidism after their first dose. When patients needed repeated treatments, the cumulative rate climbed to nearly 51%. This isn’t considered a complication so much as an expected trade-off: replacing an overactive thyroid with a controlled, medicated underactive one is often the intended goal. After surgery that removes the entire gland, hypothyroidism is guaranteed and lifelong hormone replacement becomes necessary.
Medications That Affect the Thyroid
Several widely prescribed drugs can interfere with thyroid hormone production, sometimes enough to push someone into hypothyroidism.
- Amiodarone, a heart rhythm medication, is one of the best-known offenders. It contains a large amount of iodine and disrupts thyroid function through multiple pathways: it blocks the enzymes that convert thyroid hormones into their active form, interferes with hormone entry into cells, and at higher doses can directly damage thyroid tissue through oxidative stress.
- Lithium, used for bipolar disorder, inhibits the release of thyroid hormones from the gland. People on long-term lithium therapy are routinely monitored for thyroid problems.
- Immune checkpoint inhibitors, a newer class of cancer drugs, can trigger autoimmune inflammation of the thyroid as a side effect of activating the immune system against tumors.
If you’re taking any of these medications and develop symptoms like unusual fatigue, weight gain, or cold sensitivity, thyroid function testing can determine whether the drug is affecting your thyroid. In many cases, the hypothyroidism resolves if the medication is stopped, though that decision depends on what the drug is treating.
Iodine Deficiency and Excess
Your thyroid needs iodine as a raw ingredient to build its hormones. Too little iodine is still the leading cause of hypothyroidism worldwide, particularly in regions where salt isn’t fortified with it. Adults need about 150 micrograms of iodine per day (more during pregnancy and breastfeeding). In most Western countries, iodized salt and dairy products provide enough for most people, which is why autoimmune disease has overtaken deficiency as the primary cause in those regions.
Interestingly, too much iodine can also suppress thyroid function. When the gland is suddenly flooded with iodine, it temporarily shuts down hormone production as a protective mechanism. For most people this effect is brief and self-correcting, but in those with underlying thyroid vulnerability, excess iodine can trigger lasting hypothyroidism. This is one reason amiodarone, which delivers a heavy iodine load, is so disruptive to thyroid function.
Postpartum Thyroiditis
Thyroid inflammation after pregnancy affects a significant number of new mothers and follows a distinctive pattern. It typically begins with a brief period of overactive thyroid function in the first one to four months after delivery, as the inflamed gland leaks stored hormones into the bloodstream. This is followed by a hypothyroid phase around four to eight months postpartum, when the gland’s hormone reserves are depleted and it’s too damaged to keep up with demand.
The hypothyroid phase brings low energy, cold intolerance, weight gain, and depression, symptoms that overlap heavily with the normal challenges of new parenthood and are easily overlooked. About 43% of women with postpartum thyroiditis experience only the hypothyroid phase without the initial overactive stage, making it even harder to recognize.
Most women recover normal thyroid function within three to six months. However, 20% to 50% go on to develop permanent hypothyroidism, particularly those who already have thyroid antibodies or a history of autoimmune thyroid disease.
Pituitary Gland Problems
Not all hypothyroidism starts in the thyroid itself. The pituitary gland, a pea-sized structure at the base of the brain, sends a signal called TSH that tells the thyroid how much hormone to produce. If the pituitary is damaged or dysfunctional, it may fail to send that signal, leaving the thyroid idle even though the gland itself is perfectly healthy.
This is called secondary (or central) hypothyroidism, and the most common culprit is a tumor in or near the pituitary. Head trauma, surgery in that area, and radiation therapy to the brain can also damage the pituitary enough to reduce TSH production. Secondary hypothyroidism is far less common than the thyroid-originating kind, but it’s important to identify because treatment and monitoring differ.
Congenital Hypothyroidism
Some babies are born with a thyroid that didn’t develop properly or can’t manufacture hormones normally. Congenital hypothyroidism occurs in roughly 1 in 2,000 to 1 in 4,000 newborns and is detected through routine newborn screening blood tests performed in the first days of life. In most cases, the condition is permanent and results from the thyroid gland forming incompletely during fetal development or from inherited defects in the hormone production process.
Early detection matters enormously. Thyroid hormones are essential for brain development, and untreated congenital hypothyroidism can cause irreversible intellectual disability. When caught through newborn screening and treated promptly with hormone replacement, children develop normally.
How Hypothyroidism Is Identified
Regardless of the cause, hypothyroidism is diagnosed through blood tests measuring TSH and thyroid hormone levels. In the most straightforward cases, TSH is elevated (the pituitary is shouting at the thyroid to work harder) while the actual thyroid hormone level is low. A milder form called subclinical hypothyroidism shows elevated TSH but still-normal hormone levels, meaning the thyroid is struggling but keeping up for now. Treatment is typically recommended when TSH rises above 10 mIU/L, though the decision also depends on symptoms and individual risk factors.
Because so many different conditions can cause hypothyroidism, identifying the underlying trigger often requires additional testing, such as thyroid antibodies for Hashimoto’s, imaging of the pituitary gland if central hypothyroidism is suspected, or a review of medications and dietary iodine intake. Pinpointing the cause shapes both the treatment approach and the long-term outlook.