Key Terms
Concise definitions for terms used throughout this guide
The following definitions are provided for quick reference. Full context for each appears in the relevant section of the guide.
Andropause
Age-related testosterone decline in men, typically gradual from around age 30 onward. Less abrupt and less universally timed than female menopause.
Aromatase
An enzyme, concentrated in fat tissue, that converts testosterone into oestrogen. The mechanistic basis for why excess body fat directly lowers effective testosterone levels.
DHEA-S
A stable, measurable form of the adrenal hormone DHEA, a precursor to both testosterone and oestrogen. Shows one of the steepest, most consistent age-related declines of any hormone.
Free Testosterone
The fraction of total testosterone not bound to SHBG, and therefore biologically available to tissues. Often more clinically informative than total testosterone alone.
HPG Axis (Hypothalamic-Pituitary-Gonadal Axis)
The feedback loop governing testosterone and oestrogen production, running from the hypothalamus to the pituitary to the gonads (testes or ovaries).
HPT Axis (Hypothalamic-Pituitary-Thyroid Axis)
The feedback loop governing thyroid hormone production. TSH, released by the pituitary, is the standard first-line screening measure of this axis's function.
Hypogonadism
Clinically confirmed low testosterone alongside genuine symptoms, diagnosed via repeat blood testing rather than a single reading. The condition TRT is designed to treat.
Prolactin
A hormone best known for its role in lactation that, when chronically elevated in either sex, suppresses the reproductive hormone axis and libido.
SHBG (Sex Hormone Binding Globulin)
A protein that binds testosterone and oestrogen in the blood; only the unbound ("free") fraction is biologically active. Raised by alcohol and low zinc status.
TRT (Testosterone Replacement Therapy)
Medical treatment for confirmed hypogonadism. A large 2023 trial found it non-inferior to placebo for major cardiovascular events in men with hypogonadism and cardiovascular risk, alongside some elevated risks for specific conditions like blood clots.