C h a p t e r 3 1
Mechanisms of Endocrine Control
763
DiagnosticTests
Several techniques are available for assessing endocrine
function and hormone levels. One technique measures
the effect of a hormone on body function. Measurement
of blood glucose, for example, is an indirect method of
assessing insulin levels. However, insulin action (i.e., insu-
lin activity) is also important in relation to glucose levels as
hyperglycemia could occur if marked insulin resistance is
present, despite the presence of a compensatory increase in
insulin levels (i.e., hyperinsulinemia). This situation is com-
mon in type 2 diabetes (see Chapter 33). Thus, the most
common method is to measure hormone levels directly.
BloodTests
Hormones circulating in the plasma were first detected by
bioassays using an intact animal or a portion of tissue from
the animal. However, most bioassays lack the precision,
sensitivity, and specificity to measure low concentrations of
hormones in plasma, and they are inconvenient to perform.
Blood tests provide information about hormone lev-
els at a specific time. For example, blood insulin levels
can be measured along with blood glucose after admin-
istration of a challenge dose of glucose to measure the
time course of change in blood insulin levels.
Hypothalamus
Anterior
pituitary
Releasing
hormone
Tropic hormone
Target gland
Hormone
Physiologic
effect
CNS input
Target cell
FIGURE 31-5.
Hypothalamic-pituitary control of hormone
levels.The dashed line represents feedback control. CNS,
central nervous system.
CNS
Hypothalamus
Posterior
pituitary
(oxytocin
and ADH)
Anterior
pituitary
Growth
hormone
TSH
FSH
LH
ACTH
Thyroid
(thyroid
hormones)
Adrenal gland
(glucocorticoids)
Ovaries
(estrogen and
progesterone)
Testes
(testosterone)
FIGURE 31-6.
Control of hormone production by the
hypothalamic-pituitary–target cell feedback mechanism.
Hormone levels from the target glands regulate the release
of hormones from the anterior pituitary through a negative
feedback system.The dashed line represents feedback control.
ADH, antidiuretic hormone; ACTH, adrenocorticotropic
hormone; CNS, central nervous system; FSH, follicle
stimulating hormone; LH, luteinizing hormone;TSH, thyroid
stimulating hormone.
