GLOSSARY

Steroid Hormone Transport in Blood
Steroid hormones circulate as mixtures of unbound molecules and molecules reversibly associated with plasma proteins. Transport links secretion sites with target tissues and organs that metabolize or eliminate the signal.

Different carriers

SHBG binds selected sex steroids, CBG binds glucocorticoids such as cortisol, and albumin provides broader lower-affinity binding. The relative contributions vary among steroid hormones.

Concentration and distribution

• A change in carrier abundance can alter the total circulating concentration and apparent residence time.
• It may not produce an equivalent change in the free concentration or tissue response.

Practical interpretation

Pregnancy, medicines and systemic disease can modify protein binding. During exercise, plasma-volume shifts can further alter measured concentrations. Describing a steroid result therefore requires specifying whether total or free hormone was assessed and considering the relevant transport proteins.