"A spokesman for the IOC said that the organization's belief, based on existing evidence, is that thyroid hormone used to excess "might have an anorexic or stimulant effect," but that it's more likely to inhibit than enhance performance."

Yes, 100%. I can tell you from experience that medicating with thyroid hormone to excess does NOT enhance performance. I was on 50 mcg of levothyroxin and TSH levels were good and then went off BCP. I knew that I should get my levels retested since BCP can interfere with the absorption of the medication, but procrastinated. I started experiencing muscle fatigue and soreness. My legs would just give out on rides so I could not exert any effort and would just have to spin in. I also had rapid and irregular heart beat at times. When I finally did get tested, I was well into the hyperactive range as I had been being overmedicated since going off the BCP. Now 4 weeks since starting the lower dose, I can tell a huge difference in my stamina and lack of muscle soreness and fatigue. Maybe they mean enhance performance by playing around with levels within the "normal" range, which is broad, but if you get outside of it to either extreme, which I now have done, it definitely is not beneficial in any way.