I had a hyperactive thyroid, had radioactive iodine, and now have a hypoactive thyroid. I take
replacement hormones, levothyroxine and a little triiodothyronine. Since then, 2 years ago, I have been working hard to get back in shape on my bike. My fitness has gradually increased and I've gradually been able to ride further, faster, and climb hills better. However, in the past month, when I've begun to do some long hard rides, I feel as though I've gone hypothyroid after them.