Pharmacist here. Two points: First, normal thyroid levels are a range, and as such, can certainly be manipulated, if not abused. What is normal for one person may be an athlete's hyperactivity. And there are instances in which one or more hormonal ranges may be low, but the athlete has no indication to supplement (for example, normal TSH and low T3). Second, I think the key to abuse vs. correcting a deficiency is found in whether the athlete takes T3 or T4. T4 is basically a precursor molecule, called a prohormone. Removal of one iodide converts it into T3, the active thyroid hormone that can enter the body's cells. Most people with under active thyroids take T4, with the understanding that it will be converted in the body to T3 (this is mostly how the body works; it does naturally produce both T3 and T4, but the T4 is converted to T3). Since this conversion will occur, there's no need to take T3... unless you want that immediate metabolic boost and significant fat loss. Athletes tend to abuse T3.
Here is a pretty good article:
And this explains the use of T3 and T4, although I don't believe the actual article was published or peer reviewed: