14.17: Sex Hormones in Men
- Page ID
- 84120
\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)
\( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)
\( \newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\)
( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\)
\( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\)
\( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\)
\( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\)
\( \newcommand{\Span}{\mathrm{span}}\)
\( \newcommand{\id}{\mathrm{id}}\)
\( \newcommand{\Span}{\mathrm{span}}\)
\( \newcommand{\kernel}{\mathrm{null}\,}\)
\( \newcommand{\range}{\mathrm{range}\,}\)
\( \newcommand{\RealPart}{\mathrm{Re}}\)
\( \newcommand{\ImaginaryPart}{\mathrm{Im}}\)
\( \newcommand{\Argument}{\mathrm{Arg}}\)
\( \newcommand{\norm}[1]{\| #1 \|}\)
\( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\)
\( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\AA}{\unicode[.8,0]{x212B}}\)
\( \newcommand{\vectorA}[1]{\vec{#1}} % arrow\)
\( \newcommand{\vectorAt}[1]{\vec{\text{#1}}} % arrow\)
\( \newcommand{\vectorB}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)
\( \newcommand{\vectorC}[1]{\textbf{#1}} \)
\( \newcommand{\vectorD}[1]{\overrightarrow{#1}} \)
\( \newcommand{\vectorDt}[1]{\overrightarrow{\text{#1}}} \)
\( \newcommand{\vectE}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash{\mathbf {#1}}}} \)
\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)
\( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)
\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Sources and Control of Secretion
Testosterone is the main sex steroid in men. Nearly all testosterone is secreted by the interstitial cells (Leydig's cells), which lie between the seminiferous tubules in the testes (Figure 14.1). The small amount of testosterone secreted by the adrenal cortex does not play a significant role in men unless the testes are removed. Another sex hormone, inhibin, is secreted by the sustentacular cells in the seminiferous tubules.
Secretion of testosterone and inhibin is stimulated by hormones from the anterior pituitary gland. Luteinizing hormone (LH) stimulates the interstitial cells to secrete testosterone. Because of this action, LH is also called interstitial cell-stimulating hormone (ICSH). Follicle-stimulating hormone (FSH) stimulates inhibin secretion. The secretion of all these hormones is controlled primarily by negative feedback mechanisms similar to those which regulate the secretion of thyroid hormones and glucocorticoids. Testosterone secretion can also be influenced by brain activities such as those involved in emotional reactions.
Besides stimulating inhibin secretion, FSH stimulates the sustentacular cells to manufacture a protein called androgen-binding protein (ABP), which helps testosterone stimulate sperm production by binding testosterone and concentrating it in the seminiferous tubules.
Testosterone secretion in young men occurs in a circadian rhythm. The blood level reaches its peak value during the night or early morning. Testosterone secretion and blood levels then decline during the day, reaching a minimum value by evening.
Forms of Testosterone
Much of the testosterone that passes out of the testes binds to molecules in the blood called sex hormone-binding globulin (SHBG). Testosterone that is bound to SHBG is inactive, d only free testosterone molecules significantly alter target cell activities.
Testosterone can be converted to other sex steroids. The main alternative form is dihydrotestosterone (DHT). Though the testes produce some DHT, approximately 80 percent of DHT in the blood results from the conversion of testosterone to DHT by target tissues. For example, the prostate gland releases DHT back into the blood. Some target cells respond to testosterone (e.g., skeletal muscle), while others respond to DHT (e.g., most reproductive system structures). A small amount of testosterone is converted to the hormone estrogen by certain brain regions and fat tissue.
Effects
Testosterone and DHT stimulate numerous responses in men, including (1) sperm production, (2) development and maintenance of all reproductive structures, (3) development and maintenance of male secondary sex characteristics such as deep voice, beard, thick body hair, and little fat on the hips and thighs, (4) interest in sexual activity (libido), (5) involuntary nocturnal erections during sleep, (6) thickening and strengthening of bones and muscles, and (7) a high basal metabolic rate (BMR).
Age Changes
On the average, aging is accompanied by a gradual decrease in blood testosterone levels that becomes evident after age 40 in many men. However, there is great variability in age changes in testosterone, and some older men have levels equal to or greater than the normal values for young adult men. There is also an average decrease in the proportion of free (active) testosterone. Furthermore, there is a gradual decline in the early morning peak levels, which tends to flatten the circadian rhythm, and the peaks and valleys in daily testosterone levels occur up to 2 hours later. The effects of aging on DHT levels remain controversial.
Causes of Age Changes
Age-related changes in testosterone seem to result from several age changes. These include decreasing effects of LH on interstitial cells; decreasing numbers of interstitial cells; decreasing reserve capacity for LH and FSH secretion; and changing rhythms of LH secretion. However, the age-related increase in blood levels of LH that occurs in many aging men may help compensate for these changes. The increase may also explain why less than 10 percent of older men have blood testosterone levels low enough to be considered clinically abnormal.
Other age-related factors that may reduce testosterone levels include aging of the brain; adverse changes in the circulatory system; poor nutrition; obesity; alcohol consumption; medications; institutionalization; other specific diseases; and poor general health status.
Other Factors Affecting Testosterone and DHT
Besides age-related changes in testosterone levels, men are subject to age-related changes in the effectiveness of testosterone and DHT. First, the effectiveness of testosterone is reduced by the decrease in the proportion of free testosterone. Second, the effectiveness is reduced by an age-related decrease in the number of testosterone receptors in most target cells. In contrast, the prostate gland has an age-related increase in testosterone binding, which may contribute to benign prostatic hypertrophy. Third, the effectiveness of both testosterone and DHT is reduced by the age-related increase in estrogen, which results from increased conversion of testosterone and other hormones to estrogen by fat tissue. The increase in estrogen also may be partially responsible for the age-related increase in the incidence of benign prostatic hypertrophy.
Effects of Changes in Sex Hormones
The age-related changes that affect testosterone and DHT in healthy men result in most age changes in the male reproductive system. However, testosterone levels remain adequate to sustain enough reproductive system functioning to achieve reproductive success and sexual satisfaction throughout life. Except in very old men, lower testosterone levels are not correlated with a decreased frequency of sexual activity. Furthermore, the age-related increase in blood levels of FSH and the resulting increase in stimulation of the sustentacular cells may be a compensatory factor. It may contribute to the lifelong ability to produce adequate numbers of functional sperm cells.
Using testosterone supplementation can benefit men who have a severe testosterone deficiency. Such cases are unusual. Men who have normal levels of testosterone and who take testosterone supplements receive little benefit while increasing their risks from atherosclerosis, benign prostatic hypertrophy, and possibly from prostate cancer.
Other alterations associated with age-related changes in testosterone and DHT and their effectiveness include reductions in body hair and in secretion by apocrine sweat glands and sebaceous glands. Finally, declining testosterone activity seems to be a main factor in the more rapid loss of bone matrix and the increased incidence of type II osteoporosis in older men, particularly men over age 65.