Endocrine system; Half-Life, Onset, and Duration of Hormone Activity

Hormones circulate in blood either

free or bound

• Steroids and thyroid hormone are attached to plasma proteins

• All others circulate without carriers

Concentration of circulating hormone reflects:

1. Rate of release

2. Speed at which it is inactivated and removed from body

Hormones can be removed from blood by:

• Degrading enzymes or

• Kidneys or

• Liver 

Half-life: 

• time required for level of hormone in blood level to decrease by half

  • Varies anywhere from fraction of a minute to a week, depending on hormone

Hormones have different response times:

• Some responses are immediate

• Some, especially steroid, can take hours to days

• Some are inactive until they enter target cells

The duration of hormone responsee is usually limited

• Ranges from 10 seconds to several hours

• Effects may disappear rapidly as blood levels drop, but some may persist for hours at low blood levels

Half-life, onset, and duration of hormone activity are dependent on whether

the hormone is water or lipid soluble

Multiple hormones may act on same target at same time

• Permissiveness: one hormone cannot exert its effects without another hormone being present

  • Example: reproductive hormones need thyroid hormone to have effect

• Synergism: more than one hormone produces same effects on target cell, causing amplification

  • Example: glucagon and epinephrine both cause liver to release glucose

• Antagonism: one or more hormones oppose(s) action of another hormone

  • Example: insulin and glucagon