A molecule that binds to a specific hormone that triggers alterations in cell activity.
Vitamin D is a hormone that has a half-life of one to two months. If one obtains vitamin D solely through sun (UVB) exposure during the summer months, serum vitamin D levels will be critically low by late winter. This is one reason why current recommendations are to take vitamin D supplements in order to maintain serum vitamin D levels throughout the year.
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.
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.
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.