What's Causing My Low Thyroid Level?

Medically Reviewed by Minesh Khatri, MD on January 27, 2020

Things like autoimmune diseases, surgery, and radiation treatment can all keep your thyroid gland from making the right amount of thyroid hormone.

Hashimoto's Disease

This is the most common reason that Americans have low thyroid levels. It's an autoimmune disease. If you have one, your immune system attacks your body's healthy cells. If you have Hashimoto's disease, it destroys the cells that make thyroid hormone.

Surgery on the Thyroid Gland

You can get hypothyroidism if you've had a procedure to remove part or all of your thyroid gland. You may have that done if you have a growth on your thyroid, or if it's making too much hormone (which is called hyperthyroidism).

If you have the entire gland removed, you will get hypothyroidism. For some people, if only part of it is taken out, the part that's left behind may still be able to make enough thyroid hormone.

Treatment With Radiation

This can damage cells that make thyroid hormone. You might get treated with radiation for:

  • Overactive thyroid gland
  • Cancer of the head or neck
  • Hodgkin's disease or lymphoma (cancer of the lymph system)

Thyroid Swelling

A bacterial or viral infection can make your thyroid gland swell. You may hear your doctor call this thyroiditis. The damaged thyroid leaks hormones into your blood. That causes your thyroid hormone level to rise briefly. Once that hormone gets used, your levels will drop back to normal.

Low levels from thyroiditis usually don't last long because the gland hasn't been permanently damaged.

Women can get thyroiditis after they give birth. That's called postpartum thyroiditis. It's thought to be an autoimmune disease, much like Hashimoto's disease.


Some can affect how your thyroid works and lead to low hormone levels. These include:

Lithium: Used for bipolar disorder and depression.
Interferon alpha: A cancer treatment.
Amiodarone (Cordarone, Pacerone): Given for heart rhythm problems.
Interleukin-2: Used for kidney cancer.

Too Little or Too Much Iodine

Your thyroid gland needs iodine to make its hormone. You get it from many of the foods you eat. Not getting enough is rare in the U.S. because it's added to a lot of foods. It happens more in developing countries.

Too much iodine can cause or worsen hypothyroidism.

Hypothyroidism at Birth

Some babies are born with a missing or poorly formed thyroid gland. This is congenital hypothyroidism.

Damage to the Pituitary Gland

Less often, low thyroid level comes from a problem outside the gland. The culprit may be the pituitary gland at the base of the brain, which directs your thyroid to make its hormone. If a tumor, surgery, or radiation damages your pituitary, it may not be able to give instructions to the thyroid.

Whatever the cause, medicine can get your hypothyroidism under control. Talk with your doctor about it.

Show Sources


American Thyroid Association: "Hypothyroidism," "Postpartum Thyroiditis."

Burgi, H. Clinical Endocrinology & Metabolism, February 2010.

National Endocrine and Metabolic Diseases Information Service: "Hypothyroidism."

National Health Service: "Underactive thyroid (Hypothyroid) Causes."

University of Michigan Health System: "Hypothyroidism."

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