Hormones act through two receptor classes: lipophilic steroid hormones bind intracellular receptors that regulate gene transcription; hydrophilic peptide hormones bind cell-surface receptors triggering second-messenger cascades (cAMP, IP3, calcium). The same hormone can have different effects in different tissues depending on receptor subtype and postreceptor signaling machinery. Receptor sensitivity is modulated by prior hormone exposure (desensitization) or deficiency (upregulation).
From your study of endocrine glands and second-messenger systems, you know that cells communicate chemically over distance. The central puzzle in hormone signaling is a physical one: how does a signal molecule that cannot enter a cell — or one that can — ultimately change what that cell does? The answer depends on the hormone's chemistry, and the division of hormones into two signaling strategies is one of the most organizing concepts in endocrinology.
Steroid hormones — including glucocorticoids, sex steroids, mineralocorticoids, and thyroid hormone (a structural relative) — are lipid-soluble. They diffuse freely through the plasma membrane and bind intracellular receptors, typically in the cytoplasm or nucleus. Once bound, the hormone-receptor complex acts as a transcription factor: it moves to DNA, binds specific regulatory sequences called hormone response elements, and either activates or represses target genes. The effects unfold over hours to days, because changing gene transcription takes time to produce new protein. This slow timescale matches the physiology: cortisol's metabolic effects, estrogen's effects on reproductive tissue, and thyroid hormone's effects on metabolic rate all develop gradually and persist.
Peptide hormones — including insulin, glucagon, epinephrine, and most hypothalamic and pituitary hormones — are hydrophilic and cannot cross the lipid bilayer. They bind cell-surface receptors (often GPCRs or receptor tyrosine kinases, which you studied in protein kinase signaling). Binding activates intracellular messengers: a GPCR-linked receptor may trigger adenylyl cyclase to produce cAMP, which activates protein kinase A; a receptor tyrosine kinase may autophosphorylate and recruit adaptor proteins leading to MAPK or PI3K cascades; phospholipase C activation produces IP3 (releasing calcium from the ER) and DAG (activating protein kinase C). These cascades amplify the signal enormously — one hormone molecule binding one receptor can activate thousands of enzyme molecules — and they act in seconds to minutes by modifying existing proteins through phosphorylation.
The critical concept that ties both pathways together is receptor regulation. When a cell is chronically exposed to high hormone levels, receptors are internalized and degraded — this is downregulation or desensitization, and it explains why the initial potency of a hormone fades with repeated exposure (a phenomenon relevant to drug tolerance and hormone therapies). The inverse is also true: chronic hormone deficiency leads to upregulation, increasing receptor number so the cell becomes hypersensitive to even small amounts. This dynamic regulation means that hormone levels alone do not predict cellular response — you must also know the receptor context. The same blood epinephrine concentration produces different effects in heart muscle (where β1 receptors predominate) than in bronchial smooth muscle (where β2 receptors predominate), illustrating how receptor subtype identity, not just hormone level, governs physiological outcome.