Testosterone
Testosterone is the principal male sex hormone, produced mainly in the testes, and the prescription preparations replace what the body is not making. It binds androgen receptors throughout the body and is also converted onward into oestradiol and dihydrotestosterone, which is why its effects reach beyond muscle and libido. It is prescribed for confirmed low testosterone, not for ordinary tiredness or ageing.
Replacement thickens the blood, so haematocrit and haemoglobin are checked on a schedule and a rising haematocrit is the usual reason a dose is reconsidered. Testosterone level and SHBG are followed to judge how much is actually free and active, oestradiol because some of the hormone is aromatised into it, and LDL cholesterol because androgens can shift the lipid picture unfavourably. Replacement suppresses the body's own production and sperm output, which matters for anyone who may still want children, and prostate screening continues as before rather than being dropped.
Headaches, flushing, or unusually thick-feeling blood alongside a rising haematocrit reading, and any new urinary difficulty or prostate finding, are reasons to see a doctor rather than adjust anything alone.
Reference information, not medical advice. Discuss any change with your doctor.