“T” is testosterone, the male sex hormone. It’s commonly taken by transgender people transitioning to male. Testosterone causes many of the bodily changes associated with male puberty, such as the growth of body hair and muscles. Unrelated to its medical use, it’s also taken by biological males as a performance-enhancing steroid and is on the World Anti-Doping Agency’s list of banned substances.

Anon is saying that after they started taking testosterone, they developed an “internal monologue”. Most people already have this; it’s the sound of your thoughts, such as when you lick an ice cream and think to yourself “wow, this tastes pretty good”. Some people do not have internal monologue. I do not know the medical reason for this.

Anon is expressing confusion as to why taking testosterone caused them to develop an internal monologue when they previously didn’t have one.

Edit: It is important to add context that while testosterone is a banned steroid, the World Anti-Doping agency recognises a therapeutic use exception for it, including for transgender individuals.