What causes underactive thyroid

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Last updated: April 4, 2026

Quick Answer: The most common cause of an underactive thyroid (hypothyroidism) is an autoimmune disease called Hashimoto's thyroiditis, where the body's immune system attacks the thyroid gland. Other causes include thyroid surgery, radiation therapy to the neck, certain medications, and iodine deficiency.

Key Facts

Hashimoto's thyroiditis is the leading cause of hypothyroidism in iodine-sufficient regions, affecting up to 1 in 20 people.
Thyroid surgery, particularly total thyroidectomy, results in hypothyroidism in nearly all cases.
Radioactive iodine treatment, often used for hyperthyroidism, frequently leads to hypothyroidism.
Certain medications, like lithium and amiodarone, can interfere with thyroid hormone production.
Iodine deficiency, though less common in developed countries, remains a significant cause of hypothyroidism globally.

What is an Underactive Thyroid?

An underactive thyroid, medically known as hypothyroidism, is a condition where the thyroid gland doesn't produce enough thyroid hormones. These hormones, primarily thyroxine (T4) and triiodothyronine (T3), are crucial for regulating the body's metabolism – the rate at which the body uses energy. They influence almost every organ system, affecting heart rate, body temperature, weight, energy levels, and much more. When hormone levels are too low, many of the body's functions slow down.

Common Causes of Hypothyroidism

Hashimoto's Thyroiditis: The Primary Culprit

The most frequent cause of hypothyroidism in countries where iodine intake is sufficient is an autoimmune disorder called Hashimoto's thyroiditis. In this condition, the immune system mistakenly identifies the thyroid gland as a foreign invader and launches an attack. Immune cells produce antibodies that gradually damage thyroid cells and impair their ability to produce thyroid hormones. This damage is often progressive and irreversible, leading to a lifelong deficiency in thyroid hormone production. While the exact trigger for Hashimoto's is unknown, genetic factors and environmental influences are believed to play a role.

Thyroid Surgery and Radiation Therapy

Surgical removal of all or part of the thyroid gland (thyroidectomy) is another significant cause of hypothyroidism. If the entire thyroid is removed, the body will no longer be able to produce thyroid hormones, necessitating lifelong hormone replacement therapy. Even if only part of the thyroid is removed, the remaining portion may not be sufficient to meet the body's needs. Similarly, radiation therapy directed at the neck area, often used to treat cancers of the head and neck or lymphoma, can damage the thyroid gland and lead to hypothyroidism. The effects of radiation can manifest months or even years after treatment.

Medications and Medical Treatments

Certain medications can interfere with thyroid hormone production or function. Lithium, commonly used to treat bipolar disorder, is known to sometimes cause hypothyroidism. Amiodarone, a medication used to treat irregular heartbeats, contains iodine and can disrupt thyroid hormone levels, leading to either hypothyroidism or hyperthyroidism. Interferon therapy, used to treat certain viral infections and cancers, can also affect thyroid function. If you are taking any of these medications, your doctor may monitor your thyroid function regularly.

Iodine Deficiency: A Global Concern

Iodine is an essential mineral that the thyroid gland needs to produce thyroid hormones. In regions where iodine intake from food and water is low, iodine deficiency is a common cause of hypothyroidism and goiter (enlargement of the thyroid gland). While iodized salt has significantly reduced the prevalence of iodine deficiency in many parts of the world, it remains a public health issue in certain developing countries. The World Health Organization (WHO) works globally to combat iodine deficiency disorders.

Other Less Common Causes

Other less common causes of hypothyroidism include:

Congenital Hypothyroidism: Some babies are born with an underactive thyroid. This is usually detected through newborn screening programs and requires prompt treatment to prevent developmental problems.
Pituitary Disorders: The pituitary gland, located in the brain, signals the thyroid to produce hormones. If the pituitary gland is damaged (e.g., due to a tumor or injury), it may not produce enough thyroid-stimulating hormone (TSH), leading to secondary hypothyroidism.
Thyroiditis (other forms): Besides Hashimoto's, other types of thyroiditis (inflammation of the thyroid) can temporarily or permanently impair thyroid function. This includes postpartum thyroiditis and subacute thyroiditis.
Certain Viral Infections: In rare cases, viral infections can trigger temporary hypothyroidism.

Diagnosis and Management

Hypothyroidism is typically diagnosed through blood tests that measure levels of TSH and thyroid hormones. Treatment usually involves taking a synthetic thyroid hormone replacement medication, such as levothyroxine. This medication replaces the hormone your thyroid isn't making and is generally taken for life. Regular monitoring by a healthcare professional is essential to ensure the correct dosage and manage the condition effectively.