21.4: Types of Hormones
Hormones are classified into four main groups: steroids, eicosanoids, amino acid-based derivatives, and peptide hormones.
Steroids and eicosanoids fall under the category of lipid-soluble hormones. Steroids are derived from cholesterol and feature four interconnected carbon rings with variable side chains. Notable examples include estradiol from ovaries and testosterone from testes, exemplifying the critical roles of these lipid-soluble hormones in reproductive physiology. Eicosanoids, derived from arachidonic acid, encompass prostaglandins and leukotrienes, exerting localized effects in various physiological processes like injury and infection.
In contrast, water-soluble hormones include amines, peptides, and glycoproteins. Amine hormones, synthesized from a single amino acid, include epinephrine, a tyrosine derivative commonly known as adrenaline. This hormone is pivotal in instigating the fight-or-flight response during stressful situations. Another notable example is melatonin, derived from tryptophan, which regulates the sleep-wake cycle. Peptide hormones comprise amino acid chains, such as the insulin from the pancreas. After meals, the insulin released in the blood signals cells to absorb excess glucose from the bloodstream, contributing to glucose homeostasis. Additionally, certain peptide hormones, like the thyroid-stimulating hormone, possess glycoprotein attributes featuring carbohydrate chains appended to the peptide structure.
