Chemical Classification of Hormones

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Hormones secreted by different endocrine glands vary widely in chemical structure. All hormones, however, can be divided into a few chemical classes.

1. Amines. These are hormones derived from the amino acids tyrosine and tryptophan. They include the hormones secreted by the adrenal medulla, thyroid, and pineal glands.

2. Polypeptides and proteins. Polypeptide hormones generally contain less than 100 amino acids; an example is antidiuretic hormone (table 11.2). Protein hormones are polypeptides with more than 100 amino acids; growth hormone is an example. The distinction between polypeptide and protein hormones is blurred in the case of insulin, which is composed of two polypeptide chains that are both derived from the same protein precursor.

3. Glycoproteins. These molecules consist of a long polypeptide (containing more than 100 amino acids) bound to one or more carbohydrate groups. Examples are follicle-stimulating hormone (FSH) and luteinizing hormone (LH).

4. Steroids. These are lipids derived from cholesterol. They include the hormones testosterone, estradiol, progesterone, and cortisol (fig. 11.2).

In terms of their actions in target cells, hormone molecules can be divided into those that are polar, and therefore water-soluble,

Table 11.1

A Partial Listing of the Endocrine Glands

Endocrine Gland

Major Hormones

Primary Target Organs

Primary Effects

Adipose tissue



Suppresses appetite

Adrenal cortex


Liver and muscles

Glucocorticoids influence glucose metabolism;



aldosterone promotes Na+ retention, K+ excretion

Adrenal medulla


Heart, bronchioles, and blood vessels

Causes adrenergic stimulation


Atrial natriuretic hormone


Promotes excretion of Na+ in the urine


Releasing and inhibiting hormones

Anterior pituitary

Regulates secretion of anterior pituitary hormones

Small intestine

Secretin and cholecystokinin

Stomach, liver, and pancreas

Inhibits gastric motility and stimulates bile and pancreatic juice secretion

Islets of Langerhans


Many organs

Insulin promotes cellular uptake of glucose and



Liver and adipose tissue

formation of glycogen and fat; glucagon

stimulates hydrolysis of glycogen and fat



Bone marrow

Stimulates red blood cell production




Stimulates cell division and growth


Estradiol-I7ß and progesterone

Female reproductive tract and mammary glands

Maintains structure of reproductive tract and promotes secondary sex characteristics

Parathyroid glands

Parathyroid hormone

Bone, small intestine, and kidneys

Increases Ca2+ concentration in blood

Pineal gland


Hypothalamus and anterior pituitary

Affects secretion of gonadotrophic hormones

Pituitary, anterior

Trophic hormones

Endocrine glands and other organs

Stimulates growth and development of target organs; stimulates secretion of other hormones

Pituitary, posterior

Antidiuretic hormone

Kidneys and blood vessels

Antidiuretic hormone promotes water


Uterus and mammary glands

retention and vasoconstriction; oxytocin stimulates contraction of uterus and mammary secretory units


1,25-Dihydroxyvitamin D3

Small intestine

Stimulates absorption of Ca2+




Stimulates acid secretion



Prostate, seminal vesicles, and other organs

Stimulates secondary sexual development



Lymph nodes

Stimulates white blood cell production

Thyroid gland

Thyroxine (T4) and triiodothyronine

Most organs

Thyroxine and triiodothyronine promote

(T3); calcitonin

growth and development and stimulate basal rate of cell respiration (basal metabolic rate or BMR); calcitonin may participate in the regulation of blood Ca2+ levels

Table 11.2 Examples of Polypeptide and Glycoprotein Hormones




Primary Effects

Antidiuretic hormone



Glucagon ACTH

Parathyroid hormone FSH,LH,TSH

8 amino acids 8 amino acids

21 and 30 amino acids (double chain)

29 amino acids 39 amino acids 84 amino acids Glycoproteins

Posterior pituitary Posterior pituitary Beta cells in islets of Langerhans

Alpha cells in islets of Langerhans Anterior pituitary Parathyroid Anterior pituitary

Water retention and vasoconstriction Uterine and mammary contraction Cellular glucose uptake, lipogenesis, and glycogenesis Hydrolysis of stored glycogen and fat Stimulation of adrenal cortex Increase in blood Ca2+ concentration Stimulation of growth, development, and secretory activity of target glands







Adrenal cortex



Chemical Classes Hormones

Cortisol (hydrocortisone)

Androstenedione OH

Secreted by adrenal cortex




Secreted by Leydig cells of testes


Secreted by follicles of ovaries

Interstitial (Leydig) cells

Spermatic cord

Testis Seminiferous tubules

Interstitial (Leydig) cells

■ Figure 11.2 Simplified biosynthetic pathways for steroid hormones. Notice that progesterone (a hormone secreted by the corpus luteum of the ovaries) is a common precursor of all other steroid hormones and that testosterone (the major androgen secreted by the Leydig cells of the testes) is a precursor of estradiol-17p, the major estrogen secreted by the follicles of the ovaries.

and those that are nonpolar, and thus insoluble in water. Since the nonpolar hormones are soluble in lipids, they are often referred to as lipophilic hormones. Unlike the polar hormones, which cannot pass through plasma membranes, lipophilic hormones can gain entry into their target cells. These lipophilic hormones include the steroid hormones and thyroid hormones.

Steroid hormones are secreted by only two endocrine glands: the adrenal cortex and the gonads (fig. 11.2). The gonads secrete sex steroids; the adrenal cortex secretes corticosteroids (including cortisol and aldosterone) and small amounts of sex steroids.

The major thyroid hormones are composed of two derivatives of the amino acid tyrosine bonded together (fig. 11.3). When the hormone contains four iodine atoms, it is called tetraiodothyronine (T4), or thyroxine. When it contains three atoms of iodine, it is called triiodothyronine (T3). Although these hormones are not steroids, they are like steroids in that they are relatively small, nonpolar molecules. Steroid and thyroid hormones are active when taken orally (as a pill). Sex steroids are the active agents in contraceptive pills, and thyroid hormone pills are taken by people whose thyroid is deficient (who are hypothyroid). By contrast, polypeptide and glycopro-tein hormones cannot be taken orally because they would be di-

Thyroxine, or tetraiodothyronine (T4)

Thyroxine, or tetraiodothyronine (T4)

Triiodothyronine (T3)

Triiodothyronine (T3)

■ Figure 11.3 Structural formulas for the thyroid hormones.

Thyroxine, also called tetraiodothyronine (T4), and triiodothyronine (T3) are secreted in a ratio of 9 to 1.

gested into inactive fragments before being absorbed into the blood. Thus, insulin-dependent diabetics must inject themselves with this hormone.

The pineal gland secretes melatonin, a hormone derived from the amino acid tryptophan. Melatonin has properties that are similar in some ways to both the lipophilic hormones and the water-soluble hormones. The adrenal medulla secretes the catecholamines epinephrine and norepinephrine (see fig. 9.8), which are derived from the amino acid tyrosine. Like polypeptide and glycoprotein hormones, the catecholamine hormones are too large and polar to pass through plasma membranes.

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