GLOSSARY

Hormone Receptor Occupancy
Receptor occupancy is the proportion of available receptors bound by a ligand at a specified time. It depends on the local free ligand concentration, receptor affinity and whether the system has approached binding equilibrium.

An introductory model

For independent sites with one binding class, occupancy follows free ligand concentration divided by the sum of free ligand concentration and Kd. Real endocrine receptors can show cooperativity, trafficking and multiple interacting states.

Why effect can differ

• Some systems generate a near-maximal response before every receptor is occupied because downstream signaling amplifies the input.
• In other systems, receptor numbers or signal coupling limit the response.

Applied reading

Occupancy is not measured by a routine serum hormone test. A serum result provides information about exposure, while receptor studies examine binding at the target. Distinguishing occupancy from response prevents the assumption that doubling hormone concentration necessarily doubles its physiological effect.