What is a Hormone?
Hormones are chemical messengers that are carried by the blood from endocrine glands to the cells upon which they act. The cells influenced by a particular hormone are the target cells for that hormone. Hormones can exert their specific effects in a few ways]- influencing the rate of synthesis of enzymes and other proteins
- affecting the rate of enzymatic catalysis
- altering the permeability of cell membranes

Peptide Hormone Synthesis
The majority of hormones are either peptides or proteins. Their size ranges from small peptides having only three amino-acids, to small proteins, some of which are glyco-proteins. For the sake of convenience, we follow a common practice and refer to all these hormones as peptide hormones.

In the majority of cases, peptide hormones are initially synthesized on the ribosomes of the endocrine cells as much larger proteins known as preprohormones. These are then cleaved to prohormones by proteolytic enzymes in the granular Endoplasmic Reticulum. These prohormones are then packaged into secretory vesicles by the Golgi apparatus. During this process, the pro hormone is cleaved so that an active hormone is released along with other peptide chains found in the prohormone. So, when the cell is stimulated to release the contents of the secretory vesicles by exocytosis, the other peptides are secreted. In some cases, these peptides can have certain hormonal effects. In simpler terms, sometimes instead of just one peptide hormone, a cell may be secreting multiple peptide hormones that differ in their effects on target cells.

Moreover, peptide hormones serve as both neurotransmitters and as hormones in the healthy body. Good examples can be found in the endocrine glands in the gastrointestinal tract - e.g. cholecystokinin. These peptides are also produced by neurons in the brain where they function as neurotransmitters.

Steroid Hormone Synthesis
Steroid hormones are produced by the adrenal cortex and the gonads, as well as by the placenta during pregnancy. Cholesterol is the precursor of all steroid hormones. The various biochemical steps in the synthesis of steroid hormones beyond cholesterol involve many small changes in the molecules and are mediated by specific enzymes. Therefore, the steroids that are produced by a particular cell depends on the concentrations and the types of enzymes that are present. Also, as steroids as lipid soluble, once their synthesis is completed they diffuse across the plasma membrane of the steroid producing cell and enter the interstitial fluid and then the blood.
There are five major classes of steroid hormone which are derived from cholesterol. These are]progestagens - glucocorticoids - mineralcorticoids - androgens - estrogens

Non-Peptide (Amine) Hormone Synthesis
The amine hormones are all derivatives of the amino acid tyrosine. They include the thyroid hormones, epinephrine and norepinephrine (produced by the adrenal medulla), and dopamine (produced by the hypothalamus).

Thyroid Hormones
The thyroid gland is located in the lower part of the neck wrapped around the front of the trachea. It is composed of many spherical structures called follicles, each consisting of a single layer of epithelial cells surrounding an extra-cellular central space filled with a glycoprotein colloid called thyro-globulin.

These follicles are the source of tetraiodothyronine (thyroxine, T4) and triiodothyronine (T3). T4 and T3 are synthesized from tyrosine and iodine by the enzyme complex, peroxidase. Tyrosine is incorporated in peptide linkages within the protein thyroglobulin. After iodination, two iodotyrosine molecules are coupled to yield the iodothyronines.

Secretion of T4 and T3 requires retrieval of thryoglobulin from the follicle lumen by endocytosis. To support hormone synthesis, iodide is both actively concentrated by the gland and conserved within it by recovery from the iodotyrosine that escapes secretion.

Thyrotropin (TSH) acts on the thyroid gland via its plasma membrane receptor and cAMP to stimulate all steps in the production of T4 and T3. These steps include iodie uptake, iodination and coupling, and retrieval from thyroglobulin. TSH also stimulates glucose oxidation, protein synthesis, and growth of the epithelial cells.

More than 99.5% of the T4 and T3 circulate bound to thyroid-binding globulin (TBG). Only the free fractions of T4 and T3 are biologically active. Changes in TBG levels require corresponding changes in thyroid hormone secretion to maintain normal concentrations of free T4 and T3.

T4 functions largely as a pro-hormone. Monodeiodination of the outer ring yields 75% of the daily production of T3, which is the principal active hormone. Alternatively, monodeiodination of the inner ring yields reverse T3, which is biologically inactive. Proportioning of T4 between T3 and reverse T3 regulates the availability of active thyroid hormone.

Adrenal Medullary Hormones and Dopamine
There are two adrenal glands, one on the top of each kidney. Each adrenal gland constitutes two distinct endocrine glands, an inner adrenal medulla, which secretes amine hormones, and a surrounding adrenal cortex, which secretes steroid hormones. The adrenal medulla is actually a modified sympathetic ganglion whose cell bodies do not have axons but instead release their secretions into the blood, thereby fufilling a criterion for an endocrine gland.

All adrenocorticoids are synthesized from cholesterol by sequencing enzymatic steps consisting of side chain cleavage and hydroxylation of key sites in the steroid nucleus. Cortisol specifically requires an 11-hydroxyl group, aldosterone, an 18-hydroxyl group, and androgens, a 17-hydroxyl group for respective activities. The mitochondrial and microsomal enzymes involved are P450 mixed oxygenases. Steroid hormones are not stored directly; increased secretory demands require rapid synthesis from stored cholesterol.

Cortisol and androgen secretion are regulated by adrenocorticotropin (ACTH). The pituitary hormone acts through a plasma membrane receptor with cAMP as the main second messenger. ACTH stimulates cellular uptake of cholesterol, its movement from storage vacuoles into mitochondria, and all subsequent biosynthetic steps.

The adrenal medulla, as mentioned earlier, is an enlarged, specialized sympathetic ganglion. It synthesizes epinephrine(adrenaline) and nor-epinephrine (nor-adrenaline) from tyrosine and stores these catecholamine hormones in granules. They are released in response to stimulation of preganglionic cholinergic sympathetic nervous system fibers. Hypoglycaemia, hypovolemia, hypotension, stress, and pain are major stimuli.