Thyroid problems stem from a surprisingly wide range of causes, from your own immune system attacking the gland to something as simple as getting too much or too little iodine in your diet. The thyroid is a small, butterfly-shaped gland at the front of your neck that controls metabolism, energy, and body temperature through the hormones it produces. When something disrupts that production, the effects ripple through nearly every system in your body.
Autoimmune Disease: The Most Common Cause
The immune system is behind the majority of thyroid problems in developed countries. Two autoimmune conditions account for most cases: Hashimoto’s thyroiditis and Graves’ disease. In both, the immune system mistakenly identifies parts of the thyroid as threats and mounts an attack, but the results look very different.
In Hashimoto’s thyroiditis, immune cells and antibodies gradually destroy thyroid tissue. The antibodies target two specific proteins the thyroid needs to make its hormones. Over months or years, the gland loses its ability to produce enough hormone, leading to hypothyroidism (an underactive thyroid). Symptoms come on slowly: fatigue, weight gain, cold sensitivity, dry skin, and constipation.
Graves’ disease works in the opposite direction. The immune system produces antibodies that mimic the signal your brain normally sends to the thyroid, telling it to ramp up hormone production. The thyroid responds by overproducing hormones, causing hyperthyroidism. This can trigger weight loss, rapid heartbeat, anxiety, trembling hands, heat intolerance, and more frequent bowel movements. The thyroid often visibly enlarges.
Too Little or Too Much Iodine
Your thyroid needs iodine to manufacture its hormones, and the amount matters more than most people realize. Adults need about 150 micrograms of iodine per day (220 during pregnancy, 290 while breastfeeding). Problems arise at both ends of the spectrum.
When you don’t get enough iodine, the thyroid can’t produce adequate hormones. Your brain compensates by sending stronger and stronger signals to the thyroid, essentially telling it to work harder. That persistent stimulation causes the gland to swell, producing a visible enlargement called a goiter. In mild deficiency, the enlarged thyroid may compensate well enough to keep hormone levels normal. In severe deficiency, it can’t keep up, and hypothyroidism develops. Large goiters can press on the windpipe and esophagus, making it difficult to breathe or swallow.
Excess iodine causes a different set of problems. In people who already get enough iodine, chronic intake above 1,100 micrograms per day can actually suppress thyroid hormone production, leading to hypothyroidism and goiter. The situation gets more complicated in people who were previously iodine-deficient: their thyroid may have developed autonomous nodules that, when suddenly flooded with iodine, overproduce hormones and trigger hyperthyroidism. This is particularly common in older adults with long-standing goiters.
Viral Infections and Thyroid Inflammation
A condition called subacute thyroiditis can develop a few weeks after a viral infection of the ear, sinuses, or throat, including the flu, a common cold, or mumps. The virus triggers inflammation in the thyroid gland, and the most noticeable symptom is neck pain that sometimes radiates to the jaw or ears. The gland becomes swollen and tender, and this can last for weeks or, in rare cases, months.
What makes this condition unusual is that it often causes a two-phase swing in thyroid function. First, the inflamed gland leaks stored hormone into the bloodstream, producing temporary hyperthyroidism: sweating, weight loss, nervousness, and a racing heart. As the stored hormone runs out and the damaged gland heals, it may underproduce for a while, causing fatigue, weight gain, cold sensitivity, and constipation. Most people eventually recover full thyroid function, but the process can take up to a year.
Medications That Disrupt the Thyroid
Several widely prescribed medications can interfere with thyroid function. One of the best-studied examples is amiodarone, a heart rhythm medication that contains about 37% iodine by weight. A single 200-milligram tablet delivers roughly 75 milligrams of organic iodine, which is hundreds of times the daily requirement. Up to 14 to 18% of people on long-term amiodarone therapy develop some form of thyroid dysfunction, though lower doses bring the rate closer to 4%. The excess iodine can push the thyroid toward either overproduction or underproduction, depending on whether the person had pre-existing thyroid vulnerability.
Lithium, commonly used for bipolar disorder, is another well-known culprit. It interferes with the thyroid’s ability to release its hormones, and a significant portion of people taking it long-term develop hypothyroidism. Other medications, including certain cancer immunotherapies and the antifungal drug ketoconazole, can also alter thyroid function as a side effect.
Family History and Genetic Risk
Thyroid disease runs strongly in families. More than 75% of patients with thyroid disease report that a family member on at least one side also has it, according to Cleveland Clinic endocrinologists. The more relatives affected, the higher your own risk. The genetic link often traces back to a broader tendency toward autoimmune disease. The same family might see one person with Hashimoto’s, another with type 1 diabetes, and a third with rheumatoid arthritis.
Certain inherited genetic syndromes also raise thyroid cancer risk. Cowden’s syndrome, for example, increases the likelihood of thyroid, breast, and uterine cancers. If your family has a pattern of these cancers, even without a thyroid cancer diagnosis specifically, it may point to an underlying genetic mutation worth investigating.
Radiation Exposure
The thyroid is one of the most radiation-sensitive organs in the body, especially in children. Exposure to radioactive iodine (I-131) or external radiation to the head and neck area increases the risk of both thyroid nodules and thyroid cancer. This was demonstrated in studies of atomic bomb survivors in Hiroshima and Nagasaki, as well as in people who received radiation therapy for childhood cancers or head and neck conditions.
Children face a considerably higher risk than adults because their thyroid cells are dividing more rapidly and are more vulnerable to radiation-induced damage. The adult thyroid appears much more resistant. The risk also increases with multiple exposures. Nodules or lumps may not appear until years or even decades after the original exposure.
Pregnancy and Postpartum Changes
Pregnancy places enormous demands on the thyroid. The gland needs to produce roughly 50% more hormone to support both mother and developing baby, and iodine requirements jump to 220 micrograms per day. Women with borderline thyroid function or undiagnosed Hashimoto’s may tip into overt hypothyroidism during pregnancy for the first time.
After delivery, 5 to 10% of women in the United States develop postpartum thyroiditis, an inflammatory condition that follows a pattern similar to subacute thyroiditis. A hyperthyroid phase typically appears one to four months after delivery and lasts one to three months. This is followed by a hypothyroid phase four to eight months postpartum, which can persist for nine to twelve months. Most women recover, but some remain permanently hypothyroid and need ongoing treatment. Women with type 1 diabetes or a history of thyroid antibodies are at higher risk.
Pituitary Gland Problems
Sometimes 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, sends signals telling the thyroid how much hormone to make. When the pituitary malfunctions, the thyroid doesn’t receive proper instructions.
Pituitary tumors (usually benign growths called adenomas) are the most common cause of this “central” hypothyroidism. They can compress the hormone-producing cells in the pituitary, block the communication pathway between the brain and pituitary, or, rarely, bleed suddenly in an emergency called pituitary apoplexy. Other growths near the pituitary, including cysts, meningiomas, and metastatic tumors from cancers elsewhere in the body, can produce the same effect. This type of hypothyroidism is trickier to diagnose because the usual screening test measures the pituitary’s signal rather than thyroid hormone levels directly, and results can appear misleadingly normal.