I have a physiology degree and am pretty well read on endocrinology. I read over a review paper on thyroid hormone about a year and half ago in the journal "Physiological Reviews." There were several effects of the hormone not usually mentioned that would have performance enhancing effects. Of course most people know that thyroid hormone increases metabolic rate (which would decrease percent body fat and reduce recovery time), but it also increases cardiac output, VO2 max, and increases sensitivity to epinephrine (adrenaline). Thyroid hormone also has synergistic effects with growth hormone, perhaps making growth hormone (natural or unnatural in origin) more effective. Taking a lot would not be advantageous as it would cause a lot of undesired side effects, but optimizing levels would certainly improve performance.