7.3 Hormone Action and Medical Use

Steroid hormones such as estradiol and testosterone are lipophilic. After crossing the target-cell membrane, a steroid binds a specific intracellular receptor. The complex changes shape, enters the nucleus if necessary, binds regulatory DNA and recruits transcription machinery or co-regulators. Only genes with suitable regulatory sequences in accessible chromatin respond, and only cells expressing the receptor are targets.

The response is delayed compared with direct channel opening because transcription, RNA processing, translation and protein action take time. It can persist after free hormone concentration falls because the new proteins remain. Steroid hormone action is therefore not the hormone physically 'turning into' a protein; the ligand changes the activity of a transcription-regulating receptor.

Epinephrine, also called adrenaline, travels from adrenal glands to target tissues but does not readily cross the membrane. In a liver cell it binds an adrenergic G-protein-coupled receptor, activates a G protein and stimulates adenylyl cyclase. cAMP activates protein kinase A, which phosphorylates enzymes to promote glycogen breakdown and oppose glycogen synthesis. Cascading enzymes amplify the signal while phosphodiesterases and phosphatases help terminate it.

Insulin binds the extracellular region of its receptor, activating kinase activity on the cytoplasmic side. Receptor phosphorylation creates docking sites for relay proteins, and downstream pathways promote glucose-transporter insertion in muscle and adipose membranes, glycogen synthesis and other anabolic processes. Insulin itself remains the first messenger; phosphorylated proteins relay the information.

Comparing these examples reveals why labels must be precise. Epinephrine is an amino-acid derivative that uses a GPCR and cAMP in this pathway. Insulin is a peptide using a receptor kinase. Steroids use intracellular receptors. All are hormones and all can change enzyme activity or gene expression, but the route between binding and response differs.

Recombinant human insulin produced in engineered microbes replaces a missing or insufficient signal. Synthetic growth hormone can treat a confirmed deficiency and particular growth disorders. Estrogen and progestogens are used in contraception and hormone therapy. Effective treatment depends on dose, timing, route and receptor-bearing target tissues; a hormone is not generally beneficial simply because it occurs naturally.

Use of growth hormone without medical need illustrates the difference between altered body composition and improved performance. Increased lean mass does not guarantee increased contractile strength, power or endurance, and adverse effects can occur. Evaluating a claim requires controlled outcomes, appropriate comparison groups and attention to dose—not photographs, anecdotes or a biochemical mechanism alone.