What Causes Hypothyroidism? Common Reasons for a Low or Underactive Thyroid

The thyroid is a sensitive gland. It responds to many different signals and conditions in your body. It can also be vulnerable to injury from chronic medical conditions, medications, and even nutritional deficiencies. These things can lead to decreased thyroid hormone levels, and low thyroid function, leading to hypothyroidism. 

So, if you have recently been diagnosed with hypothyroidism — or you’re wondering if your thyroid hormone levels are low — it helps to understand which factors affect thyroid function. Let’s take a closer look at eight possible causes of hypothyroidism. 

1. Autoimmune thyroiditis (Hashimoto’s disease)

Autoimmune thyroiditis, also called Hashimoto’s disease, happens when your immune system makes proteins called anti-thyroid peroxidase (anti-TPO) antibodies  These antibodies lead to inflammation of your thyroid gland. They attack the thyroid the same way your immune system would attack viruses or bacteria. 

Over time, this can damage your thyroid gland and lower how much hormone it makes. This condition is most common in women during midlife, but it can happen at any age. 

2. Treatments and medications for other medical conditions

Sometimes, treatments for other conditions can result in hypothyroidism. These treatments include:

• Thyroidectomy: This is the surgical removal of part or all of your thyroid. It’s used to remove thyroid nodules, goiters, or cancer. 

• Thyroid ablation: This treatment for an overactive thyroid (called hyperthyroidism) uses radioactive iodine to destroy parts of your thyroid. Sometimes, it can cause your thyroid to make too little hormone.

• Radiation: People who get radiation for cancer near the thyroid — like the head, neck, or chest — may have some radiation absorbed into their thyroid. This can damage thyroid tissue.

Medications can also cause hypothyroidism. Common ones include methimazole and propylthiouracil, which are both used to treat hyperthyroidism. Like an ablation, they can also lower hormone levels too much. Other medications that can cause hypothyroidism include: 

• Immunotherapy medications

• Lithium

• Amiodarone

• Ethionamide

3. Iodine deficiency

Your thyroid gland needs iodine to make thyroid hormones. But your body doesn’t make iodine on its own, so you have to get it from the foods you eat. 

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If you don’t get enough iodine in their diet, your thyroid can’t make enough thyroid hormone. When this happens, your thyroid cells work in overdrive to try to make more. Hypothyroidism from iodine deficiency can cause a goiter — an enlarged thyroid gland.

Hypothyroidism from iodine deficiency is more common in developing countries. In the U.S., table salt usually has iodine added to it. So, it’s rare to see iodine deficiency as a cause of hypothyroidism in this country. However, it’s important to keep in mind that designer salts — like pink Himalayan salt — usually don’t have iodine added to it.

4. Subacute thyroiditis

Temporary hypothyroidism can happen when you have subacute thyroiditis. This is usually caused by a viral infection and can affect anyone. It causes mild viral symptoms — like fever, fatigue, and body aches — and then thyroid swelling. This swelling can either be painful or painless. 

Subacute thyroiditis can cause abnormal thyroid hormone levels — high or low — but this isn’t permanent. Subacute thyroiditis can last weeks to months, but it eventually goes away.

5. Riedel’s thyroiditis

Riedel’s thyroiditis is a rare type of thyroid inflammation. For unknown reasons, scar tissue forms in the thyroid and replaces healthy tissue. This leads to hypothyroidism, since scar tissue can’t produce thyroid hormone like a healthy thyroid.

6. Infiltration

Infiltration occurs when diseases that affect other parts of your body also affect your thyroid. These diseases can damage your thyroid and stop it from making enough thyroid hormone. In these cases, healthy thyroid tissue is invaded — or infiltrated — by non-thyroid tissue that doesn’t work the same way.

Examples of these infiltrative diseases are:

• Amyloidosis

• Sarcoidosis

• Lipomatosis

• Langerhans cell histiocytosis

• Scleroderma

• Hemochromatosis

7. Congenital hypothyroidism

Some babies are born with an underactive or underdeveloped thyroid. In most cases, the thyroid gland is either missing, smaller than it should be, or in the wrong place. Any of these can lead to low thyroid hormone levels.

It’s usually diagnosed in infants at birth, and affects girls more than boys. If left untreated, hypothyroidism can cause many health problems in infants. This is why most countries test babies for thyroid issues on the second or third day after birth.

8. Secondary hypothyroidism

Secondary hypothyroidism — also called central hypothyroidism — happens when your thyroid gland doesn’t get the right signals from your brain. The problem isn’t your actual thyroid gland, but faulty signaling tells your thyroid gland to produce less hormone. 

Think of it like a TV that won’t turn on because the remote control is broken. The TV works, but it can’t get the signal it needs. In this case, the thyroid is the TV, and the brain is the remote.

Your brain sends signals to your thyroid through two main parts: your pituitary gland and hypothalamus. Secondary hypothyroidism can happen if there’s a problem in your brain that affects these areas. This includes:

• Tumors or growths

• Infections

• Strokes that affect the pituitary gland or hypothalamus

• Damage from radiation therapy 

Medications can also get in the way of this signaling pathway. These include:

• Growth hormone therapy

• Certain corticosteroids

• Somatostatin analogues

• Bexarotene

• Morphine

What are the symptoms of hypothyroidism?

Some people with hypothyroidism don’t have any symptoms. When people do have symptoms, they can be vague or subtle, like:

• Fatigue (feeling tired)

• Weight gain

• Cold intolerance (feeling cold when others aren’t)

• Dry skin

• Constipation

• Irregular menstrual cycles

• Muscle aches

Different types of hypothyroidism have different causes. So, they can vary in how they feel and how they’re treated. 

Finding the cause of hypothyroidism

When you see a healthcare professional about a new diagnosis of hypothyroidism, here’s what to expect during your visit:

• A head-to-toe review of your symptoms

• A detailed physical exam, with a focus on your thyroid gland

• A review of your blood work

• A discussion about other testing that you might need to get the diagnosis right

Every person is unique, and sometimes a diagnosis can be difficult to make. Not every case fits into the definitions discussed here. 

But almost all treatments for hypothyroidism are the same no matter the cause. If your body isn’t making enough thyroid hormone, a healthcare professional will prescribe you replacement thyroid hormone.

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Written by:

Farah Naz Khan, MD

Farah Naz Khan, MD, is a board-certified physician at the UW Medicine Diabetes Institute and a clinical assistant professor of metabolism, endocrinology, and nutrition at the University of Washington.

Edited by:

Karla Robinson, MD

Karla Robinson, MD, is a medical editor for GoodRx. She is a licensed, board-certified family physician with almost 20 years of experience in health through varied clinical, administrative, and educational roles.

Reviewed by:

Maria Robinson, MD, MBA

Maria Robinson, MD, MBA, is a board-certified dermatologist and dermatopathologist who has practiced dermatology and dermatopathology for over 10 years across private practice, academic, and telehealth settings. She is a fellow of the American Academy of Dermatology and the American Society of Dermatopathology.

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