The Causes of an Underactive Thyroid

Updated April 17, 2017

If you or someone you know has been diagnosed with an underactive thyroid, you may wonder what caused the condition. In many cases, having an underactive thyroid is due to an inherited condition. However, there are several other reasons the thyroid may be underactive. Here, you'll learn the most common causes of an underactive thyroid.

Hashimoto's Thyroiditis

The most common cause for having an underactive thyroid is an autoimmune condition called Hashimoto's thyroiditis. The body's immune system is meant to attack outside threats, such as bacteria. However, sometimes something goes wrong, and the body attacks its own tissues. In Hashimoto's disease, the immune system slowly destroys the thyroid gland, and a sufferer must take thyroid hormone replacement pills. The condition has a strong hereditary component. Having a close relative with the condition increases your chances of developing the condition. It is also much more common in women than in men.

Treatment for Hyperthyroidism

When someone has an overactive thyroid and experiences the symptoms of hyperthyroidism, she might opt for radioactive iodine treatments or surgery to remove part of the thyroid. After the treatment, the patient's thyroid transitions to being underactive, and hormone replacement is usually needed.

Medication Side Effect

Thyroid function can be lowered as a side effect of some medications. One of the medications well known to have this effect is the psychiatric drug lithium. Perhaps as many as one in five people taking lithium will experience a decrease in her thyroid function. The lithium both interferes with body's ability to create thyroid hormone and with the thyroid gland's ability to release thyroid hormone. For some, discontinuing lithium therapy will restore thyroid function. Others may become permanently hypothyroid after treatment with lithium.

Congenital Condition

Some babies are born with low thyroid function because of a missing or abnormal thyroid. Nutritional deficiencies--including a lack of iodine in the mother's diet--may make a child hypothyroid at birth. Children with missing or defective thyroid glands must take hormone replacement therapy for life. If the condition isn't caught soon after birth, the child will suffer developmental disabilities and mild to severe retardation.


If someone has surgery on or near the thyroid gland, it may be damaged, causing the person to develop an underactive thyroid. For instance, if nodules or cancer cells are being removed from the thyroid, it may result in enough loss of thyroid tissue to make the person suffer from an underactive thyroid.

Viral Infection

In rare cases, a viral infection may cause the thyroid gland to become inflamed and enlarged. In most cases, once the illness and inflammation subsides, the thyroid regains full function. In few cases, the thyroid is permanently damaged, and thyroid hormone replacement therapy is needed.

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About the Author

Sarah Rigg has a Bachelor of Arts degree in English and philosophy from Western Michigan University. She taught technical writing at WMU for several years and has been writing and editing for more than a decade. Rigg won awards for her creative writing and for her work at community newspapers.