If you have myasthenia gravis, your risk of developing thyroid disease is higher than that of the general population. Studies have found that between 5 percent and 10 percent of people with myasthenia gravis also have thyroid issues.

Fortunately, even though thyroid disease is common in people with myasthenia gravis, it’s very treatable when diagnosed and treated early.

What Does Your Thyroid Do?

Your thyroid is a butterfly-shaped gland found at the front of your neck, below your Adam’s apple. The thyroid gland is part of your endocrine system — the organs responsible for making and releasing hormones. The hormones your thyroid releases are called thyroid hormones.

Thyroid hormones control metabolism, which is the process your body uses to turn food into energy. The two thyroid hormones, thyroxine (also known as T4) and triiodothyronine (T3), tell your body how much energy to use.

Your pituitary gland helps control the levels of thyroid hormones in your blood. If your levels are too low, your pituitary gland will release thyroid-stimulating hormone (TSH), which tells your thyroid gland to make more thyroid hormone.

When your thyroid is working well, your metabolism will make and burn energy at the right rate. If this gland isn’t working properly, you may have either hyperthyroidism (too much thyroid hormone) or hypothyroidism (not enough thyroid hormone).

How Is Thyroid Disease Related to Myasthenia Gravis?

Thyroid issues are relatively common, affecting an estimated 5 percent of people in the United States, according to a 2023 study in the journal Endocrine Practice. On the other hand, myasthenia gravis is rare, occurring in around 14 to 20 in 100,000 people, per the Myasthenia Gravis Foundation of America. People with myasthenia gravis are more than two times as likely to develop a coexisting thyroid disease than the general population, and the risk increases over time.

Scientists believe that thyroid diseases may be related to myasthenia gravis because both conditions stem from problems with the immune system. Research has found that people with myasthenia gravis and autoimmune thyroid disorders may have some genes in common. Environmental factors, such as viral infections and certain medications, may also trigger myasthenia gravis or thyroid problems.

Myasthenia gravis and some thyroid diseases are autoimmune disorders. Autoimmune disorders happen when your body makes antibodies (specialized immune system proteins) that mistakenly attack healthy tissue, which are called autoantibodies.

Myasthenia gravis is caused by antibodies attacking acetylcholine receptors (AChRs) in the neuromuscular junction, the space where your nerves and muscles meet. When antibodies attack and destroy AChRs, your nerves have trouble communicating with your muscles.

Some thyroid diseases are caused by antibodies attacking the thyroid, which can lead to either hyperthyroidism or hypothyroidism.

1. Graves’ Disease

Graves’ disease, the most common type of thyroid disease that occurs with myasthenia gravis, is a type of autoimmune thyroid disease that causes hyperthyroidism. People with Graves’ disease make an antibody called thyroid-stimulating immunoglobulin (TSI). TSI attaches to your thyroid and tells it to make more thyroid hormone.

Too much thyroid hormone (also called thyrotoxicosis) can cause several body functions to speed up. Possible symptoms of Graves’ disease include:

• Rapid heart rate
• Heart palpitations (irregular heartbeat)
• Nervousness or irritability
• Anxiety
• Weight loss
• Excessive sweating
• Insomnia (trouble falling asleep or staying asleep)
• Hair loss or brittle hair
• Changes to the menstrual cycle
• Diarrhea
• Enlarged thyroid gland

Graves’ disease can also cause symptoms similar to myasthenia gravis, such as muscle weakness. If Graves’ disease affects your eyes, they may appear as if they’re bulging out of your face. Other eye and vision problems similar to ocular myasthenia gravis include:

• Diplopia (double vision)
• Blurred vision
• Dry eyes
• Swollen, puffy eyes
• Light sensitivity
• Pressure or pain in your eyes

There’s a high prevalence of Graves’ disease in people with ocular myasthenia gravis and in those with mild generalized myasthenia gravis.

Graves’ Disease Treatment

Graves’ disease can be treated with antithyroid medications, radioiodine therapy, or surgery called thyroidectomy to remove the thyroid gland. It’s possible that treating hyperthyroidism due to Graves’ disease may worsen some myasthenia gravis symptoms. However, eye- and vision-related symptoms in people with both myasthenia gravis and Graves’ disease usually improve after treatment for Graves’ disease.

2. Hashimoto’s Thyroiditis

Hashimoto’s thyroiditis (also called Hashimoto’s disease) is another type of autoimmune thyroid disease that can occur in people with myasthenia gravis. People with Hashimoto’s thyroiditis have hypothyroidism. Low thyroid hormone levels are caused by antibodies that attack healthy thyroid tissue (also known as antithyroid antibodies). When antibodies attack your thyroid, it results in damaging inflammation that prevents your thyroid from making enough thyroid hormones.

Common symptoms of Hashimoto’s thyroiditis include:

• Fatigue
• Weight gain
• Difficulty tolerating cold temperatures
• Slow heart rate
• Hair loss or brittle hair and nails
• Dry skin
• Depression
• Constipation
• Heavy or irregular menstrual periods
• Fertility problems
• Pain in joints and muscles
• Concentration difficulties
• Dysphonia (hoarseness)

Although Hashimoto’s thyroiditis isn’t as common as Graves’ disease in people with myasthenia gravis, it may be associated with more severe myasthenia gravis. For a study published in 2019, researchers in Thailand observed people with myasthenia gravis and thyroid disorders for 20 years. Their findings showed that people with Hashimoto’s thyroiditis needed more aggressive treatment for myasthenia gravis, and myasthenia gravis was more often associated with thymoma (tumor in the thymus gland).

Hashimoto’s Thyroiditis Treatment

Hashimoto’s thyroiditis is most commonly treated with a synthetic (laboratory-made) version of T4 thyroid hormone called levothyroxine.

3. Thyroid Antibodies With Normal Thyroid Hormone Levels

If you have antithyroid or TSI antibodies, you may still have normal thyroid hormone levels — known as being euthyroid — and may not experience any symptoms. In a 2017 study, researchers found that myasthenia gravis is associated with all types of thyroid issues, even if thyroid hormone levels are normal.

Treatment for Thyroid Antibodies

You may not need treatment for thyroid antibodies if you have normal thyroid hormone levels. Instead, your doctor may follow up with you to monitor your thyroid hormone levels and make sure they aren’t too high or too low.

4. Thyroid Cancer

Cancer that starts in the thyroid is also linked with myasthenia gravis. Symptoms of thyroid cancer include:

• A lump in your neck
• Difficulty swallowing
• Difficulty breathing
• Hoarse voice
• Swollen lymph nodes in your neck

Thyroid Cancer Treatment

Treatment for thyroid cancer may involve the following options:

• Surgery to remove all or part of the thyroid gland
• Radioiodine therapy
• Radiation therapy
• Chemotherapy
• Hormone therapy

5. Goiter

A goiter is a lump in your neck caused by an enlarged thyroid gland. Any of the previously mentioned thyroid issues (Graves’ disease, Hashimoto’s thyroiditis, and thyroid cancer) can lead to goiter, as can iodine deficiency, pregnancy, and thyroid inflammation.

Goiters are classified as toxic or nontoxic. A toxic goiter results in too much thyroid hormone, whereas a nontoxic goiter doesn’t affect your thyroid hormone levels. People with more severe types of myasthenia gravis may have an increased risk of toxic goiters.

Goiter Treatment

Treatment for goiter is directed at its underlying cause, such as Graves’ disease or Hashimoto’s thyroiditis.

Talk to Your Doctor

Talk to your doctor about your risk of developing thyroid issues. If you already have thyroid issues, your doctor can help you understand your best treatment plan, based on your symptoms and risk factors.

Talk With Others Who Understand

On MGteam, the social network for people living with myasthenia gravis and their loved ones, members come together to ask questions, give advice, and share their stories with others who understand life with myasthenia gravis.

Are you living with myasthenia gravis? Are you experiencing any issues with your thyroid? Share your experience in the comments below, or start a conversation by posting on your Activities page.