Steroid hormones are bioactive compounds that have a specific ring configuration. These hormones are derived from cholesterol. Examples of steroidal hormones are estrogens, testosterone, progesterone, thyroid hormones, and cortisol. These hormones are transported in the body using transporter proteins because they are water-insoluble. Steroid hormones may cross the plasma membrane to bind to the nucleus or to a nuclear receptor.
Peptide and amino acid hormones like epinephrine, any pituitary or hypothalamic hormones, and insulin are water-soluble hormones. These hormones are stored in vesicles and the endoplasmic reticulum of endocrine cells until needed. Amino acid hormones function by binding to protein receptors on the outside surface of the cell. This binding creates a second messenger molecule cascade inside the cell that activates enzymes and other cellular proteins or influences gene expression.
Amino acid and polypeptide hormones are generally hydrophilic as they "like" water and are considered to be water-soluble hormones that travel freely in the blood. Water-soluble hormones generally bind to extracellular targets on the plasma membrane.
Sex hormones (estrogens, testosterone, and progesterone), cortisol, and thyroid hormones are fat-soluble hormones and are water-repellent/resisting (hydrophobic). Fat-soluble hormones "like" lipid or fatty structures such as those structures surrounding cells, like cell and nuclear membranes. Fat-soluble, water-insoluble steroid hormones need special protein carriers for transportation throughout the body. Albumin is the most commonly used plasma protein for transporting lipid hormones.
