A personal anecdote: recently I came quite close to being diagnosed with CFS or fibromyalgia but luckily found out I have primary hyperparathyroidism. I have no idea how all the doctors could have missed it, I had to figure it out myself.
"Of all the missed diagnoses of ME/CFS and fibromyalgia, primary hyperparathyroidism proves to be one of the most tragic simply as a result of how treatable the condition is relative to the suffering it can cause." https://phoenixrising.me/archives/23988
"Of all the missed diagnoses of ME/CFS and fibromyalgia, primary hyperparathyroidism proves to be one of the most tragic simply as a result of how treatable the condition is relative to the suffering it can cause." https://phoenixrising.me/archives/23988