Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 25The Gonads: Development & Function of the Reproductive System 407

increased formation of cAMP via the G protein-coupled LH
receptor and Gs. Cyclic AMP increases the formation of choles-
terol from cholesteryl esters and the conversion of cholesterol to
pregnenolone via the activation of protein kinase A.

Secretion

The testosterone secretion rate is 4 to 9 mg/d (13.9–31.33
μmol/d) in normal adult males. Small amounts of testosterone
are also secreted in females, with the major source being the
ovary, but possibly from the adrenal as well.

Transport & Metabolism

Ninety-eight percent of the testosterone in plasma is bound to
protein: 65% is bound to a β-globulin called gonadal steroid-
binding globulin (GBG) or sex steroid-binding globulin,
and 33% to albumin (Table 25–5). GBG also binds estradiol.
The plasma testosterone level (free and bound) is 300 to 1000
ng/dL (10.4–34.7 nmol/L) in adult men (Figure 25–8), com-
pared with 30 to 70 ng/dL (1.04–2.43 nmol/L) in adult women.
It declines somewhat with age in males.
A small amount of circulating testosterone is converted to
estradiol, but most of the testosterone is converted to 17-keto-
steroids, principally androsterone and its isomer etiocholano-
lone (Figure 25–16), and excreted in the urine. About two
thirds of the urinary 17-ketosteroids are of adrenal origin, and
one third are of testicular origin. Although most of the 17-
ketosteroids are weak androgens (they have 20% or less the
potency of testosterone), it is worth emphasizing that not all
17-ketosteroids are androgens and not all androgens are 17-
ketosteroids. Etiocholanolone, for example, has no andro-
genic activity, and testosterone itself is not a 17-ketosteroid.

Actions

In addition to their actions during development, testosterone
and other androgens exert an inhibitory feedback effect on
pituitary LH secretion; develop and maintain the male secon-

dary sex characteristics; exert an important protein-anabolic,

growth-promoting effect; and, along with FSH, maintain

spermatogenesis.

Secondary Sex Characteristics

The widespread changes in hair distribution, body configura-

tion, and genital size that develop in boys at puberty—the

male secondary sex characteristics—are summarized in

Table 25–6. The prostate and seminal vesicles enlarge, and the

seminal vesicles begin to secrete fructose. This sugar appears

to function as the main nutritional supply for the spermato-

zoa. The psychic effects of testosterone are difficult to define

in humans, but in experimental animals, androgens provoke

boisterous and aggressive play. The effects of androgens and

estrogens on sexual behavior are considered in detail in

TABLE 25–5 Distribution of gonadal steroids and
cortisol in plasma.

% Bound to

Steroid % Free CBG GBG Albumin

Testosterone 2 0 65 33

Androstenedione 7 0 8 85

Estradiol 2 0 38 60

Progesterone 2 18 0 80

Cortisol 4 90 0 6

CBG, corticosteroid-binding globulin; GBG, gonadal steroid-binding globulin.
(Courtesy of S Munroe.)

FIGURE 25–16 Two 17-ketosteroid metabolites of

testosterone.

TABLE 25–6 Changes at puberty in boys (male

secondary sex characteristics).

External genitalia: Penis increases in length and width. Scrotum

becomes pigmented and rugose.

Internal genitalia: Seminal vesicles enlarge and secrete and begin to

form fructose. Prostate and bulbourethral glands enlarge and secrete.

Voice: Larynx enlarges, vocal cords increase in length and thickness,

and voice becomes deeper.

Hair growth: Beard appears. Hairline on scalp recedes anterolaterally.

Pubic hair grows with male (triangle with apex up) pattern. Hair ap-

pears in axillas, on chest, and around anus; general body hair increases.

Mental: More aggressive, active attitude. Interest in opposite sex develops.

Body conformation: Shoulders broaden, muscles enlarge.

Skin: Sebaceous gland secretion thickens and increases (predisposing

to acne).

H

Etiocholanolone

O

O

Androsterone

H

HO

HO