He uses a "hairsystem."
He may have taken some more mild measures between 1997-2003 to arrest the decline in hair density that began in OKC era, but these efforts- the kind known to all of us, and advertised widely in the media- were only sufficient not to look "bald," they didn't bring back the ability to rock a full head of hair, which is why he had closely-cropped haircuts in that era, it was the only way to look decent.
After HTTT, as he hit middle age, he decided he wanted a more youthful head of hair. Like many stars who undergo cosmetic surgeries during a long hiatus in their career (hoping it will pass unnoticed, being interpreted as natural aging or a mere change in their style) he started to take more drastic measures (whose nature is mysterious, but a company making "hairsystems" has taken credit) between HTTT and IR era.
That also is in line with IR's midlife crisis themes, its focus on romantic fantasy and its abandonment of punk rock inspirations (apart from one song). Thom "sold out" his beliefs in one way in 1991 (signing to a major record corporation) and the next 12 years of his work were a brilliant reaction against that, and then just as he extricated himself to go indie, he "sold out" in another way (changing his appearance to fit social norms and others' desires, giving in to the temptation of vanity) which is what his work since then has been reacting against.
If Thom didn't see himself as a sellout in some way, he would not be artistically driven and brilliant as he is. To that extent, the hairsystem was win-win. It gave him more confidence in his personal life, but also created a new source of guilt that fed into his music and returned him to a very raw personal focus, just at the moment his inspiration might've dried up. The hairsystem company should really take credit for In Rainbows.