**2.1 General male reproductive hormones**

Male reproductive functions are mediated by different hormones [3]. The 'master' regulator hormonal axis is the hypothalamo-pituitary-gonadal/testicular axis which is led by the pulsatile release of hypothalamic gonadotropin-releasing hormone (GnRH). This, in turn, stimulates anterior pituitary trophic hormones \_ luteinizing hormone (LH) and follicle-stimulating hormone (FSH). Luteinizing hormone acts upon the testicular cells, the Leydig cells for steroidogenesis while follicle stimulating hormone acts on the Sertoli cells to aid spermatogenesis. This primary axis is influenced by an array of other testicular hormones, metabolic hormones and different regulatory factors. These hormonal cross-talks influence the intricate testicular functions, sexual behavior and semen quality in men, including semen volume [3]. See **Figure 1** for the illustration.

Testes secrete steroids known as androgens. These androgens, in turn, are responsible for maintenance of male characteristics as well as male behaviors. Follicle stimulating hormones (FSH) released from pituitary gland stimulates the growth of the seminiferous tubules. Seminiferous tubules constitute much of the

#### **Figure 1.**

*Hypothalamo-pituitary-testicular Axis and its crosstalk with other hormones in the regulation of male reproductive functions [3].*

#### *Testosterone DOI: http://dx.doi.org/10.5772/intechopen.94017*

structure of the testes. The hormone, FSH, promotes within seminiferous tubules cell divisions which will result in the production of mature sperm [4].

The luteinizing hormones (LH), also released from pituitary gland, promotes the development of certain endocrine tissues within the testes. These tissues are composed of groups of cells (interstitial tissue) between the seminiferous tubules.

Under the influence of LH, the interstitial tissues secrete the steroid hormone testosterone. Even though testosterone may be secreted by the adrenal cortex, the amount of secretion is usually insignificant [4]. Testosterone is also produced in ovary in females [5].

Testosterone, in the presence of normal amounts of growth hormone, promotes growth of the bony skeleton. This is in addition to promoting male characteristics, male behaviors and the maintenance of the spermatic tubules [4, 6, 9]. The secretion of androgen markedly increases at puberty accounting for rapid growth at that stage of life [4, 7].

Illustration is shown in **Figure 1**.

### **2.2 Testosterone**

Testosterone is the primary male sex hormone and anabolic steroid [6]. It is the main anabolic steroid hormone produced by the body but serving two main effects on the body:


Some athletes take testosterone to boost their performance because of its anabolic effects [6].

Testosterone is produced majorly by the testicles in adult human \_ men [7]. In women, the ovaries produce the hormone [8]. The hormone affects a man's appearance and sexual development, stimulates sperm production and regulates a man's sex drive, including semen volume. It also helps build muscles and bone masses [7, 8, 11].

Testosterone production decreases with age. Its production is at its highest in a man's early adulthood and drops slowly each year afterwards [9]. The normal range of testosterone in the body is typically 300 to 1000 ng/dL (nanogram per deciliter). A range of symptoms can occur if its production drastically drops below normal. Men with low testosterone level, also known as 'low T', can experience a range of symptoms if its decrease becomes significant. Low testosterone, or low T, is diagnosed when levels fall severely below 300 ng/dL in male or 15 ng/dL in female. A blood test, called 'a serum testosterone test', is usually used to determine the level of circulating testosterone [7, 9]. When the body does not produce the right amount of testosterone, the condition is called hypogonadism. This is sometimes called "low T". Men diagnosed with hypogonadism can benefit from testosterone therapy. However, therapy is not usually recommended, unless testosterone level falls quite below the normal range for a given age. This is because there are some natural remedies which can help [9].

#### *2.2.1 Normal and abnormal levels of testosterone*

A normal testosterone level range for men is 300 to 1000 nanograms per deciliter (ng/dL) [7]. For women, its normal level is between 15 and 70 ng/dL. However, it's also considered normal to have changes in the level of testosterone throughout life [8].

#### *Reproductive Hormones*

Testosterone levels can decrease naturally due to age or other health conditions. After the age of 40, men's testosterone levels decrease. The decrease, on average, is about one percent (1%) every year. When men reach above 40 years of age, some symptoms of low testosterone become commonly seen. The commonest symptom to appear is erectile dysfunction. Irrespective of age, low testosterone levels have often been observed in people with obesity [8].

Hypogonadism, also called low testosterone, is the most common problem related to testosterone, in men.

As discussed earlier (Section 2.2), Testosterone is responsible for traits such as body hair, muscle mass and strength. Therefore, men with low levels of testosterone might notice a reduction in these traits whereas women with too much testosterone might notice an increase in these traits [8].

#### *2.2.2 Chemical properties of testosterone*

The molecular formula is **C19H28O2** [10]. The molecular weight is 288.42 g/mol [11]. In medical treatment, the therapeutic testosterone is a synthetic form of endogenous androgenic steroid testosterone [12].

Generally speaking, in vivo, testosterone is converted into dihydrotestosterone (DHT) in the target tissues irreversibly by the enzyme 5-alpha reductase. The testosterone or DHT ligand-androgen receptor complexes then act as transcription factor complexes. In this way, it stimulates the expression of various responsive genes. In comparison, DHT binds with higher affinity to androgen receptors than testosterone, thus activating gene expression more efficiently. Testosterone, in addition, is irreversibly converted to estradiol by the enzyme complex aromatase. This occurs particularly in the liver and adipose tissue. Both testosterone and DHT promote the development and maintenance of male sex traits related to the internal and external genitalia, skeletal muscle and hair follicles. On the other hand, estradiol promotes epiphyseal maturation and bone mineralization. However, because of rapid metabolism by the liver, therapeutic testosterone is generally administered as an ester derivative (**Figure 2**) [11].

**Figure 2.** *Chemical structure of testosterone [5].*
