steroid hormone

Meaning: Steroid hormones are derived from cholesterol and share a four-ring carbon skeleton. They are nonpolar, so they diffuse through the phospholipid bilayer of target cells. Inside, they bind receptors in the cytoplasm or nucleus, and the hormone-receptor complex acts as a transcription factor that binds DNA and alters gene expression. Because new mRNA and protein must be made, onset is slow (often hours) and effects tend to last longer. Being hydrophobic, they travel in blood bound to carrier proteins. Examples come from the adrenal cortex (cortisol, aldosterone) and the gonads (estrogen, progesterone, testosterone).

On the MCAT: Passages contrast steroid and peptide hormones. Peptides are water-soluble, bind cell-surface receptors, and typically act fast through second messengers such as cAMP and G proteins. Expect questions asking which hormone needs a membrane receptor, which acts by changing transcription, or what happens if a transcription or protein-synthesis inhibitor is added (steroid effects are blunted, while rapid peptide effects are not). Steroid-producing cells are rich in smooth ER, and these hormones are generally made on demand rather than stored in vesicles.

Common confusion: Students assume every lipid-soluble hormone is a steroid. Thyroid hormones also use intracellular receptors and alter transcription, but they are derived from tyrosine, not cholesterol, so they are not steroids.