Rheumatoid arthritis and hypothyroidism often occur together. Researchers don't know why autoimmune diseases travel in packs, but they can be successfully treated.

Autoimmune diseases travel in packs. If you have one, you're at risk for developing others.

For example, people with rheumatoid arthritis are also more likely to develop an underactive thyroid disease — hypothyroidism or Hashimoto’s thyroiditis. And people with Hashimoto's are also at higher risk for developing rheumatoid arthritis.

Affecting about 1.5 million Americans, rheumatoid arthritis, or RA, occurs when the body’s immune system misfires against its own joints and connective tissues, causing pain and inflammation.

Similarly, Hashimoto’s occurs when the immune system mistakenly attacks thyroid cells, interfering with their ability to make thyroid hormone. Hypothyroid symptoms such as fatigue, joint and muscle pain, and depression can also mimic RA symptoms, making it tricky to sort out which condition is responsible for which symptoms.

Exactly how the two conditions are linked is not yet fully understood, but shared genes may play a role in susceptibility to autoimmune diseases, according to some research. One new study showed that people taking medication to treat hypothyroidism are twice as likely to have RA. The risk increased if individuals tested positive for certain genes linked to RA. The findings appeared in the Annals of Rheumatic Disease.

RA + Hypothyroidism = Heart Disease?

Eric L. Matteson, MD, chair of the division of rheumatology and professor of medicine at Mayo Clinic in Rochester, Minn. said heart disease is also more prevalent among individuals with both hypothyroidism and RA, or more than one autoimmune condition.

“It’s the trifecta,” said Dr. Matteson. “When you have an underactive thyroid, it can make you feel quite tired, just like RA. Everyone assumes it is the RA, so nobody thinks to look for alternate causes.”

He added that frequently when a patient has both autoimmune hypothyroidism and RA, it can be difficult to determine which condition is causing which symptoms.

Some overlapping symptoms include swelling around the feet and legs, muscle discomfort, and weight gain. “If someone with RA has unusual new fatigue that is getting worse and weight gain, these symptoms and signs should make us think that their thyroid is not functioning properly, and it’s reasonable to check thyroid function with blood tests,” he said. The reassuring news is that thyroid medication won’t interfere with RA medication, so both conditions can be treated simultaneously and successfully, he added.

Yael Tobi Harris, MD, an endocrinologist with the North Shore-LIJ Health System in Great Neck, N.Y., said the diseases can emerge in any order or happen simultaneously. “Someone may have RA and be treated, and then their rheumatologist may do blood work and find an underactive thyroid," said Dr. Harris. "Or someone may have hypothyroidism and develop worsening joint pain, and they may subsequently get diagnosed with RA.

Often, blood tests are the only way to figure out which condition is driving the symptoms. But, cautioned Harris, most people with hypothyroidism don’t have RA. "We don’t routinely screen for RA in the absence of symptoms," she said.