Testosterone

Testosterone is a C19 sex steroid hormone, that may be synthesized from:

• androstenedione e.g. in the adrenal cortex
• cholesterol in the Leydig cells of the testes
• oestradiol
• 17-ketosteroids

Actions of testosterone include:

• the features of pubarche in puberty
• anabolism
• maintenance of gametogenesis; in the male the final maturation of spermatids to spermatozoa requires the action of testosterone on the Sertoli cells
• negative feedback on secretion of luteinizing hormone by the pituitary

Transport:

• 60-70% of testosterone is bound to sex hormone binding globulin (SHBG); the majority of the remaining testosterone is loosely bound to albumin, alpha1 acid glycoprotein and transcortin
• 2% of testosterone is not protein-bound and therefore can enter cells to bind to intracellular androgen receptors

Testosterone can be further converted in certain tissues, via the enzyme 5-alpha reductase, into its more potent metabolite, dihydrotestosterone. Also some testosterone is converted into oestradiol in the liver, adipose tissue and the CNS (1). Also testosterone is converted to inactive metabolites in the liver, adipose tissue, muscle and kidney. Renal excretion is facilitated via conjugation of such metabolites to glucoronides in the liver.

Testosterone secretion in males follows a circadian rhythm, with a peak at 7-9 am and trough in late afternoon.

Reference:

• (1) Testosterone and other androgens: physiology, pharmacology, and therapeutic use. In DeGroot LJ (Ed). Endocrinology. 2010.