Midterm 2 - Importance of Thyroid Hormones Flashcards
where is the thyroid gland located
both side of the trachea, below the larynx
what are the 2 lobes of the thyroid gland bridged by
isthmus
what are characteristic of the thyroid gland
heavily vascularized
heaviest of pure endocrine glands
what is the thyroid gland composed of
numerous follicles
what are follicles in the thyroid gland
a single layer of epithelial cells that release thyroid hormones
lumen filled with protein rich fluid (colloid)
what occurs between follicles in the thyroid gland
parafollicular cells (C-cells) secrete calcitonin
how much of circulating thyroid hormones does TBG bind to
70-80% - remaining binds to other albumins
what percentage of T3 and T4 are free in blood
0.03%
why is there several days of hormone available bound to TBG
only 0.03% free in blood and diffuse to tissues - only free hormone is active
what are free and bound forms in
equilibrium
what cells do thyroid hormones act on
all cells of the body
what do thyroid hormones act on all cells to do
increase basal metabolic rate by increasing heat production and O2 consumption
where does O2 consumption not increase due to thyroid hormones
brain
gonads
spleen
what do TH stimulate during development
GH
what can a TH deficiency lead to
dwarfism - TH is important for bone and tissue growth
when are TH critical for normal maturation of brain function
during the last phase of foetal development and early stage after birth
what can a lack of TH lead to
cretinism
what do TH increase
the number of adrenalin/noreadrenalin receptors –> boost sensitivity to SNS
what are TH essential for
gonadal normal function
what does a lack of TH in gonad lead to
reduced sperm produciton
irregular cycles
how does TH effect reflexes
increases nerve conduction velocity –> faster reflexes
what is hypothyroidism mainly due to in humans
iodine deficiency
what can hypothyroidism result in
reduced growth and mental retardation
what is hypothyroidism mainly due to in dogs
autoimmune disorders that destroy follicular cells = primary hypothyroidism
what do lab tests show in dogs with hypothyroidism
low T4 and T3 with high TSH
what is secondary hypothyroidism due to
low secretion of TSH, mainly due to injury to pituitary or hypothalamus
what cell types are affected by hypothyroidism
most
symptoms of hypothyroidism
reduced heat production and tolerance to cold
sluggish animal, not feeding but gaining weight
impaired reproduction
constipation
treatment for hypothyroidism
thyroxine for the rest of life
what is a goiter
enlarged thyroid gland
what are causes of goiter
iodine deficiency
tumor and inflammation
lack of thyroid hormones increases TRH and TSH release (lack of feedback) which stimulates multiplication (hyperplasia) and volume (hypertrophy) of follicular cells
what area of animal are gaiters more frequent in
inland - coastal air, vegetation, and marine organisms are a good source of iodine
hyperthyroidism
over-production of thyroid hormones
what animal is hyperthyroidism frequent in
cats - unknown cause
what does hyperthyroidism target
many target organs
symptoms of hyperthyroidism
big appetite associated with weight loss
excitable and nervous
increased heart rate, respiration and digestive passage
treatment of hyperthyroidism
surgery, radioactive I2 to destroy follicular cells
what is Tgb
a glycoprotein with over 120 tyrosine residues
