Suppose that the circulating concentration of hormone is 10^–10 M and the Kd for binding to its receptor is 10^–8 M. What fraction of the receptors will have hormone bound? If a meaningful physiological response occurs when 50% of the receptors have bound a hormone molecule, how much will the concentration of hormone have to rise to elicit a response? The fraction of receptors (R) bound to hormone (H) to form a receptor–hormone complex (R–H) is [R–H]/ ([R] + [R–H]) = [R–H]/[R]_TOT = [H]/([H] + K_d).