STRUCTURES AND FUNCTIONS
Hormones exert their effects on target tissue.
The specificity of hormone–target cell interaction is determined by receptors in a “lock-
and-key” type of mechanism.
The regulation of hormone levels in the blood depends on a highly specialized
mechanism called feedback.
With negative feedback, the gland responds by increasing or decreasing the secretion of
a hormone based on feedback from various factors.
The hypothalamus and pituitary gland integrate communication between the nervous and
Several hormones secreted by the anterior pituitary are referred to as tropic hormones
because they control the secretion of hormones by other glands.
o Thyroid-stimulating hormone (TSH) stimulates the thyroid gland to secrete thyroid
o Adrenocorticotropic hormone (ACTH) stimulates the adrenal cortex to secrete
o Follicle-stimulating hormone (FSH) stimulates secretion of estrogen and the
development of ova in the female and sperm development in the male.
o Luteinizing hormone (LH) stimulates ovulation in the female and secretion of sex
hormones in both the male and female.
Growth hormone (GH) has effects on all body tissues.
Antidiuretic hormone (ADH) regulates fluid volume by stimulating reabsorption of
water in the kidneys.
Oxytocin stimulates ejection of milk into mammary ducts and contraction of uterine
The major function of the thyroid gland is the production, storage, and release of the
thyroid hormones, thyroxine (T ) and triiodothyronine (T ).
o Iodine is necessary for the synthesis of thyroid hormones.
o T a4d T af3ect metabolic rate, carbohydrate and lipid metabolism, growth and
development, and nervous system activities.
Calcitonin is a hormone produced by C cells (parafollicular cells) of the thyroid gland in
response to high circulating calcium levels. Parathyroid Gland
The parathyroid glands are four small, oval structures usually arranged in pairs behind
each thyroid lobe. They secrete parathyroid hormone (PTH), which regulates the blood
level of calcium.
The adrenal glands are small, paired, highly vascularized glands located on the upper
portion of each kidney.
o The adrenal medulla secretes the catecholamines epinephrine (the major hormone
[75%]), norepinephrine (25%), and dopamine.
o The adrenal cortex secretes cortisol, which is the most abundant and potent
glucocorticoid. Cortisol is necessary to maintain life. Its functions include
regulation of blood glucose concentration, inhibition of inflammatory action, and
support in response to stress.
o Aldosterone is a potent mineralocorticoid that maintains extracellular fluid volume.
o The adrenal cortex secretes small amounts of androgens. Adrenal androgens
stimulate pubic and axillary hair growth and sex drive in females.