Thyroid problems stem from a surprisingly wide range of causes, from your own immune system attacking the gland to nutritional gaps, medications, pregnancy, and even environmental chemicals. The single most common cause worldwide is autoimmune disease, where the body’s defense system mistakenly targets thyroid tissue. Understanding what triggers thyroid dysfunction can help you recognize risk factors you may already carry and make sense of a diagnosis if you’ve recently received one.
Autoimmune Disease: The Leading Cause
The two most common thyroid disorders are both autoimmune. In Hashimoto’s thyroiditis, immune cells infiltrate the thyroid and slowly destroy the tissue that produces hormones. Specifically, certain white blood cells invade the gland, causing inflammation and killing the hormone-producing cells. Over time, the damaged tissue is replaced by scar tissue, and the gland gradually loses its ability to make enough thyroid hormone. The result is hypothyroidism, an underactive thyroid.
Graves’ disease works in the opposite direction. Instead of destroying the thyroid, the immune system produces antibodies that latch onto receptors on thyroid cells and force them to overproduce hormones. These antibodies mimic the signal the brain normally sends to regulate the thyroid, essentially jamming the accelerator. The result is hyperthyroidism: too much thyroid hormone flooding the body, speeding up heart rate, metabolism, and a host of other functions.
Both conditions run strongly in families. A large population study in Sweden found that if you have a first-degree relative (parent or sibling) with Graves’ disease, your risk is roughly four times higher than average. For Hashimoto’s, the risk is nearly five times higher. When both a parent and a sibling are affected, the numbers jump dramatically: over 11 times the risk for Graves’ and 22 times for Hashimoto’s. If autoimmune thyroid disease runs in your family, it’s worth monitoring your thyroid function periodically even before symptoms appear.
Iodine: Too Little or Too Much
Your thyroid needs iodine to build its hormones. Adults require about 150 micrograms per day, roughly what you’d get from iodized salt, dairy, seafood, or eggs. Pregnant women need significantly more, around 220 mcg, and breastfeeding women need 290 mcg. In many parts of the world where iodine is scarce in the diet, deficiency remains the leading cause of preventable thyroid problems, particularly goiter (an enlarged thyroid gland) and hypothyroidism.
In countries where salt is iodized, outright deficiency is uncommon but not impossible, especially in people who avoid iodized salt, eat very restricted diets, or are pregnant with increased demands. Paradoxically, consuming too much iodine can also suppress thyroid function. The gland has a built-in safety mechanism that temporarily shuts down hormone production when iodine floods in. For most people this is a short-lived response, but in those with underlying thyroid vulnerability, it can tip the balance into hypothyroidism.
Medications That Affect the Thyroid
Several widely prescribed drugs can interfere with thyroid function. Two of the most well-known offenders are a heart rhythm medication called amiodarone and the mood stabilizer lithium.
Amiodarone is loaded with iodine. As the body metabolizes the drug, large amounts of iodide are released, which can overwhelm the thyroid’s normal production process. The drug also interferes with enzymes that help convert thyroid hormones into their active form, and it disrupts the feedback loop between the brain and the thyroid. The net effect can go either way: some people develop hypothyroidism, others develop hyperthyroidism. Thyroid monitoring is standard for anyone taking this medication.
Lithium works differently. It increases the iodine content inside the thyroid while simultaneously blocking the final steps of hormone assembly and release. The gland has the raw materials but can’t get the finished product out. Over time, this can lead to an underactive thyroid in a significant percentage of people on long-term lithium therapy. Certain cancer immunotherapy drugs and targeted therapies can also cause thyroid dysfunction, so thyroid levels are routinely checked during those treatments as well.
Thyroiditis: Inflammation With a Pattern
Thyroiditis refers to inflammation of the thyroid gland, and it can be triggered by viral infections, autoimmune flare-ups, or childbirth. What makes thyroiditis distinctive is that it often follows a predictable three-phase pattern: an initial burst of hyperthyroidism (as stored hormone leaks from damaged cells), followed by a dip into hypothyroidism (as the gland struggles to recover), and finally a return to normal function.
Subacute thyroiditis typically follows a viral upper respiratory infection. Viruses implicated include Epstein-Barr, influenza, adenovirus, and SARS-CoV-2. The overactive phase usually lasts three to six weeks, and about one-third of patients then slide into a hypothyroid phase that can persist for up to six months. Most people recover fully within 12 months.
Silent thyroiditis is an autoimmune variant that follows a similar pattern but without the pain or viral trigger. It can occur in anyone but is most commonly identified in women.
Postpartum Thyroiditis
Pregnancy reshapes the immune system in ways that can unmask thyroid problems. Postpartum thyroiditis affects roughly 5 to 10 percent of women within the first year after delivery, miscarriage, or medical abortion. During pregnancy, the immune system dials down to protect the fetus. After delivery, it rebounds, sometimes aggressively enough to attack the thyroid.
About 40 percent of women with postpartum thyroiditis present with hypothyroidism alone. Another 25 to 40 percent experience the classic three-phase swing: a mild hyperthyroid phase starting two to six months after delivery, lasting two to three months, followed by hypothyroidism between three and 12 months postpartum. Most women return to normal thyroid function within three to six months of the hypothyroid phase. However, a subset go on to develop permanent hypothyroidism, particularly if they had thyroid antibodies before or during pregnancy.
Thyroid Nodules and Structural Changes
Thyroid nodules are extremely common. Imaging studies suggest that somewhere between 20 and 76 percent of the general population has at least one, though the vast majority are too small to feel and cause no symptoms at all. The wide range in that estimate reflects the sensitivity of the imaging method used: high-resolution ultrasound picks up tiny nodules that would never be found on a physical exam.
Most nodules are benign. Thyroid cancer is found in 5 to 15 percent of nodules that are biopsied, meaning the overwhelming majority are harmless. That said, nodules that grow, cause difficulty swallowing, or show suspicious features on ultrasound are typically evaluated with a fine-needle biopsy. A nodule that produces excess thyroid hormone on its own, sometimes called a “hot” nodule, can cause hyperthyroidism even when the rest of the gland is functioning normally.
Pituitary and Brain-Related Causes
The thyroid doesn’t operate independently. It takes orders from the pituitary gland, a pea-sized structure at the base of the brain that releases a signaling hormone (TSH) telling the thyroid how much hormone to make. When the pituitary is damaged, that signal weakens or disappears, and the thyroid slows down even though the gland itself is perfectly healthy. This is called central or secondary hypothyroidism.
The most common cause is a pituitary adenoma, a benign tumor that compresses the hormone-producing cells or disrupts blood flow between the brain and the pituitary. Traumatic brain injury, pituitary surgery, and radiation therapy to the head can also damage this signaling pathway. Central hypothyroidism is much rarer than primary thyroid disease, but it’s worth knowing about because standard screening tests can sometimes miss it. The TSH level, which is elevated in primary hypothyroidism, may appear deceptively normal or only mildly low in central cases.
Environmental Chemicals
A growing body of evidence links environmental contaminants to thyroid disruption. Perchlorate, a chemical found in rocket fuel, fireworks, and some fertilizers, can interfere with the thyroid’s ability to absorb iodine. At high exposure levels, it blocks iodine uptake into the gland, potentially reducing hormone production. The U.S. FDA has monitored perchlorate levels in the food supply for this reason, since trace amounts can end up in water and certain foods.
Other industrial chemicals, including certain flame retardants, pesticides, and per- and polyfluoroalkyl substances (PFAS, sometimes called “forever chemicals”), have also been investigated for thyroid-disrupting effects. These compounds can interfere with hormone transport, metabolism, or the receptors that thyroid hormones act on. For most people, everyday exposure levels are low, but the concern is greater for those with existing thyroid vulnerability, iodine deficiency, or occupational exposure.
Other Risk Factors
Sex is one of the strongest risk factors for thyroid disease. Women are five to eight times more likely than men to develop thyroid problems, particularly autoimmune forms. The reasons aren’t fully understood but likely involve hormonal and genetic factors related to the X chromosome.
Age also plays a role. Hypothyroidism becomes more common with age, and thyroid antibodies are found more frequently in older adults. Radiation exposure to the neck, whether from medical treatment (such as radiation therapy for head and neck cancers) or environmental events, increases the risk of both hypothyroidism and thyroid cancer. A history of other autoimmune conditions, including type 1 diabetes, rheumatoid arthritis, or celiac disease, also raises the likelihood of developing autoimmune thyroid disease.