Hypothyroidism — an underactive thyroid — affects roughly 5 percent of the adult population in the United States alone. When your thyroid gland fails to produce enough hormones, nearly every system in your body can slow down, leading to fatigue, weight gain, and brain fog. Understanding the most common hypothyroidism causes is the first step toward getting the right diagnosis and treatment.
1. Hashimoto’s Disease (Autoimmune Thyroiditis)
Hashimoto’s disease is by far the leading cause of hypothyroidism in the United States and other iodine-sufficient countries. In this autoimmune condition, your immune system mistakenly attacks healthy thyroid tissue. Over time, the ongoing inflammation damages the gland so severely that it can no longer produce adequate hormones. According to NIH, Hashimoto’s disease is the most common cause of hypothyroidism, and it tends to run in families. Women are affected far more often than men. If you have a family history of autoimmune disorders, talk to your doctor about screening your thyroid antibody levels.
2. Iodine Deficiency
Your thyroid needs iodine — a trace mineral found in seafood, dairy, and iodized salt — to manufacture hormones. When your diet lacks sufficient iodine, the gland simply cannot keep up with demand. Research indicates that iodine deficiency remains the most common cause of hypothyroidism on a global scale, particularly in developing regions. As noted by PubMed, autoimmune thyroid diseases lead in iodine-sufficient areas, while iodine deficiency dominates elsewhere. If you suspect your iodine intake is low, a simple dietary review with a nutritionist can help identify gaps.
3. Thyroid Surgery
Surgical removal of part or all of the thyroid gland is a well-known cause of hypothyroidism. Doctors may recommend thyroidectomy for thyroid cancer, large goiters, or overactive thyroid that doesn’t respond to other treatments. When a significant portion of the gland is removed, the remaining tissue may not produce enough hormones to meet your body’s needs. In cases of total thyroidectomy, lifelong thyroid hormone replacement is necessary. If you’re scheduled for thyroid surgery, ask your endocrinologist what your post-operative hormone plan will look like.
4. Radiation Treatment
Radioactive iodine therapy, commonly used to treat hyperthyroidism and certain thyroid cancers, can permanently damage thyroid cells. Radiation directed at the head or neck area for other cancers — such as lymphoma or head and neck tumors — can also harm the thyroid gland. The damage may appear months or even years after treatment ends. Studies suggest that a significant percentage of patients receiving radiation to the neck region eventually develop hypothyroidism. Regular thyroid function tests after radiation therapy can catch a decline early and allow timely intervention.
5. Certain Medications
Several commonly prescribed drugs can interfere with thyroid hormone production. Lithium, widely used for bipolar disorder, is one of the most well-documented offenders. Amiodarone, a heart medication, contains large amounts of iodine and can disrupt normal thyroid function in either direction. According to PubMed, drugs are a recognized cause of primary hypothyroidism alongside autoimmune thyroiditis and iodine imbalances. Immune checkpoint inhibitors used in cancer treatment have also emerged as a newer cause. Always inform your doctor about every medication you take so thyroid levels can be monitored appropriately.
6. Congenital Hypothyroidism
Some babies are born with a thyroid gland that is absent, underdeveloped, or positioned incorrectly. This condition, known as congenital hypothyroidism, occurs in roughly one out of every 2,000 to 4,000 newborns. Without early detection, it can lead to serious developmental delays and intellectual disability. Fortunately, most countries now include thyroid screening in routine newborn blood tests. When caught early and treated with daily thyroid hormone, children with congenital hypothyroidism can develop normally. Parents should make sure their newborn’s screening results are reviewed promptly.
7. Thyroiditis (Thyroid Inflammation)
Thyroiditis refers to inflammation of the thyroid gland, and it can take several forms. Postpartum thyroiditis affects some women within the first year after giving birth. Silent thyroiditis and subacute thyroiditis can follow viral infections. In many cases, the gland initially releases excess stored hormone, causing a brief hyperthyroid phase. This is often followed by a hypothyroid phase as the gland’s reserves become depleted. While some forms resolve on their own, others lead to permanent underactive thyroid. If you experience unexplained fatigue or neck tenderness after illness or pregnancy, a thyroid panel is worth requesting.
8. Pituitary Gland Disorders
The pituitary gland, a pea-sized structure at the base of your brain, produces thyroid-stimulating hormone. When the pituitary malfunctions, it may fail to signal the thyroid properly. This form is called secondary or central hypothyroidism, and it accounts for a small but important fraction of cases. Clinical evidence shows that pituitary adenomas and other pituitary or hypothalamic dysfunctions are common triggers of central hypothyroidism. Surgery, radiation, or tumors affecting the pituitary can all play a role. Diagnosis requires specialized testing beyond a standard TSH check, so mention any relevant symptoms to your doctor.
9. Excess Iodine Intake
While too little iodine starves the thyroid, too much can paradoxically shut it down as well. This phenomenon, known as the Wolff-Chaikoff effect, occurs when high iodine levels temporarily block hormone production. Sources of excess iodine include certain supplements, contrast dyes used in medical imaging, and seaweed-heavy diets. People with underlying autoimmune thyroid disease are especially vulnerable to iodine-induced hypothyroidism. The effect is usually temporary, but repeated exposure can lead to lasting dysfunction. Before taking iodine supplements, have your levels tested to avoid tipping the balance in the wrong direction.
10. Infiltrative Diseases
Rare infiltrative conditions can physically invade the thyroid gland and disrupt its function. Diseases such as amyloidosis, sarcoidosis, and hemochromatosis may deposit abnormal substances in thyroid tissue. This gradually replaces healthy cells with non-functional material. Iron overload from hemochromatosis, for example, has been linked to impaired thyroid hormone production in clinical research. Because these conditions are uncommon, they are sometimes overlooked during a standard workup. If you have a known infiltrative disease and notice symptoms like persistent fatigue or cold sensitivity, ask your physician to check your thyroid function.
Hypothyroidism has many potential triggers, ranging from autoimmune attacks to medication side effects and rare infiltrative diseases. Knowing these causes empowers you to have more informed conversations with your healthcare provider. If you suspect your thyroid may be underperforming, schedule a comprehensive thyroid panel — early detection makes effective management much easier.
Medical Disclaimer: This article is for informational purposes only and does not constitute medical advice. Always consult a qualified healthcare professional before making health decisions.
