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Posterior Pituitary and Thyroid Gland (Thyroid Gland (hormone synthesis…
Posterior Pituitary and Thyroid Gland
Thyroid Gland
most common endocrinopathy in dogs and older cats
stimulated by thyroid stimulating hormone
hormones
constructed from tyrosine
poorly soluble in water
99% of hormones bound to thyroxine-binding globulin while circulating in blood
types
thyroxine (T4)
triiodothyronine (T3)
primary active hormone (bioactive)
mostly comes from T4 de-iodination
have profound influence on nearly every tissue in the body
impact development, growth, and stimulate metabolic activities
hormone synthesis and release
tyrosine stored as thyroglobulin in the colloid
iodide taken up from the blood by thyroid epithelial cells and transport it to colloid
thyroid peroxidase causes iodination of tyrosine on thyroglobulin
synthesis of T4 (or T3)
thyroid hormones accumulate in the colloid, still attached to thyroglobulin
colloid endocytosed
thryglobulin is digested by lysosome enzymes
thyroid hormone diffuses out into the blood
regulation
regulation of TRH and TSH secretion
synthesis of mostly T4
neg feedback inhibition of TRH and TSH by thyroid hormones
conversion of T4 to T3
inactivation of thyroid hormones
pathophysiology
hypothyroidism
common in old dogs
iodine deficiency: causes goiter
thyroid becomes large due to increased TRH secretion caused by decreased thyroid hormone secretion
primary thyroid disease: autoimmune - destroys part of thyroid
hyperthyroidism
common in old cats
likely due to goitrogenic chemicals in food or environment
signs: weight loss with increased eating, PU/PD, nervous/aggressive, mid-high T4, low TSH
Posterior Pituitary
antidiuretic hormone (ADH, vasopressin)
synthesized in hypothalmic neurons
bound to carrier protein - neurophysin
targets the kidney
antidiuretic
decreases rate of urine formation
receptors
V1
vascular smooth muscle, liver, platelets, CNS
contraction of smooth muscle
increased vascular resistance
increased blood pressure
V2
causes movement of aquaporins to allow reabsorption of water into cells/circulation
increases water permeability
V3
on corticotropic cells
cause ACTH secretion
regulation
stimulatory
increases solute concentration
angiotension II
hypoglycemia
inhibitory
decreased solute concentration
osmolarity below threshold = ADH suppression
hypervolemia
neg feedback (V3 receptors)
pathophysiology
diabetes insipidus
central
polyuria due to insufficient ADH
nephrogenic
ADH secretion is normal but renal insensitivity to antidiuretic efects
dipsogenic
excessive fluid intake reflects abnormal perception of thirst
oxytocin
secreted from ovaries and testes
released during sex, childbirth, and lactation
stimulates maternal behavior and maternal aggression
stimulates milk ejection
stimulates contraction of uterine smooth muscle
facilitates sperm transport
regulation
stimulatory
suckling, cervical dilation, sight/sound/smell of infant
inhibitory
acute stress