Oh wow. My sister, a fitness trainer, had hyperthyroidism. She'd wake up complaining that her heart was palpatating and she'd get lightheaded. A physically fit woman in her early 50's, she was diagnosed around 6 years ago. She opted for radiation and now she has no functioning thyroid at all. It killed it. Now she takes Synthroid, a synthetic hormone which the the thyroid was responsible for producing. She feels much better and still teaches aerobic classes 3 times a week, works out with weights and power walks 5 miles a day.

This disorder is often passed down in families.