The number of hormone molecules available for complex formation is usually the key factor that determines the level at which signal transduction pathways are activated. The number of hormone molecules that are available is determined by the concentration of circulating hormones.
Half-Life
The blood levels of hormones reflect a balance between synthesis/secretion and degradation/excretion. The liver and kidneys are the major organs that degrade hormones with breakdown products excreted in urine and feces.
A hormone's half-life and duration of activity are limited and vary from hormone to hormone. For instance, the biological half-life of luteinizing hormone is 20 minutes, which is shorter than that of a follicle-stimulating hormone (three to four hours), and of human chorionic gonadotropin (24 hours).
A biological half-life or elimination half-life is the time it takes for a substance such as a hormone or drug to lose half of its pharmacologic or physiologic activity. In a medical context, half-life may also describe the time it takes for the blood plasma concentration of a substance to halve (plasma half-life) its steady-state.
The relationship between the biological and plasma half-lives of a substance can be complex, due to factors including their accumulation in tissues, active metabolites, and receptor interactions.
Duration
The duration of hormone activity refers to the duration of events that were stimulated by hormone-receptor binding. While typically relatively short and measured in minutes or hours, certain events, such as the onset of puberty, are much longer lasting.
Hormone levels during menstrual cycle
This image depicts the levels of certain hormones during the menstrual cycle (B), as they correspond to follicular growth and ovulation (A). 1. Follicle-stimulating hormone 2. Estrogen 3. Luteinizing hormone 4. Progesterone.