Testosterone

Testosterone is a hormone belonging to the category of androgens (typically male sex hormones).

Its functions, in puberty and adulthood, are:

Acquisition of secondary sexual characteristics (appearance of beard, mustache, hair, temporal recession, lowering of the tone of the voice)
More aggressive attitudes
Libido development
Muscle mass increase

In adulthood, specifically, he is responsible for:
hair growth, maintenance of secondary sexual characteristics, and the possible appearance of baldness.

Testosterone helps fight gender dysphoria through hormone replacement therapy.
When a female biological individual begins this transition, she undergoes gradual and largely irreversible defeminization.
Androgen induction therapy for male characteristics is also able, in a short time (2-3 months), to eliminate the most feminine event of these subjects, menstruation, to the satisfaction of the patient.

In general, FTM (Female to Male) are divided into Pre-t and On-T precisely considering whether or not they are in HRT.