Heather Rae El Moussa has been diagnosed with Hashimoto's disease

She told TODAY.com: "I remember saying to my assistant, 'I feel like I'm dead'. My brain was so tired. My body was so tired. I was exhausted all the time and no amount of sleep could make it better. Filming was absolutely brutal because I could barely get out of bed."

"I was like, 'I'm probably just foggy because of mom brain'. When [my doctor] told me what I had, I was in total shock."

According to The Mayo Clinic: "Hashimoto's disease is an autoimmune disorder affecting the thyroid gland. An autoimmune disorder is an illness caused by the immune system attacking healthy tissues. In Hashimoto's disease, immune-system cells lead to the death of the thyroid's hormone-producing cells.

"Symptoms can include fatigue and sluggishness, increased sleepiness, muscle aches tenderness and stiffness, problems with memory or concentration, hair loss...." and more.

Since starting to take medicine for the condition, Heather, 36, has been feeling "much better".

While Tristan is her first child, Heather is also stepmother to 42-year-old Tarek's children Brayden, eight, and Taylor, 13, whom he shares with ex-wife Christina Hall.

Dr. Terry Davies, a professor of medicine and director of the division of endocrinology at Mount Sinai Hospital in New York City told PEOPLE: "Hashimoto's is inflammation of the thyroid gland, and it's a very common disease. It comes in mild and more severe forms, and it's an autoimmune disease.

"When the thyroid starts to fail - and the thyroid drives everything in the body, there's nothing that doesn't need the thyroid hormone - a low thyroid can lead to weight gain usually, can lead to depression, can lead to high cholesterol. It's a very slow onset, and so when the patient comes to the doctor you're never quite sure when the disease started. It could be a couple years ago, it could be quite recent, as opposed to other diseases where it's quite sudden."