thyroid gland Flashcards
thyroid gland secretes:
- thyroid hormones (T3, T4)
- calcitonin
what are the key features of the endocrine system
- release hormones into intercellular or perivascular CT spaces
- cells arranged in clusters or cords
- associated with dense capillary beds and wide thin-walled vessels called sinusoids
what are follicles
- functional units of thyroid gland
- spheres of low cuboidal to high columnar cells
- centers filled with colloid
what are follicular cells
- acidophilic
- polar cell morphology
- apical microvilli
- golgi, ER
- vaculoes
- large membrane-bound colloid droplets
what do follicular cells store
pro-hormone in follicle lumen
what do follicular cells secrete
active thyroid hormone (T3 and T4) into pericapillary space
what do follicular cells concentrate
iodine from blood
what is iodine essential for
T3/T4 synthesis
what can an iodine deficiency cause
goiter
what is thyroglobuoin
- glycoprotein
- synthesized by ER/golgi complex and secreted via secretory vesicles into follicle lumen
where is thyroperoxidase and what does it do
- on apical surface
- iodinates thyroglobulin on tyrosines
where is iodinated thyroglobulin taken up
follicular lumen
what is thyroglobulin broken down into
- thyroxine (T4) and triiodothyronine (T3)
how much of the circulating T3 is formed in the liver
83%
what is thyroxine (T4)
- pro-hormone
- converted to T3 by target cells and liver
what is thyronamines (TAMs)
family of decarboxylated and deiodinated metabolites of T3 and T4
what are the three TH distributor proteins
- thyroxine-binding globulin
- transthyretin
- albumin
what do TH distributor proteins form
a buffering network for free T4 in blood - helps protect against hypothyroidism or hyperthyroidism
what kind of thyroid hormones enter target cells
only free T4 and T3
describe the HPT axis
- TRH from hypothalamus stimulates adenohypophysis to release TSH
- TSH stimulates thryoid follicular cells
what are goitrogens
substances that interfere with thyroid hormone production
what happens if there is too much iodine
diminish T3/T4 hormone production
what is triiodothyronine (T3)
- accelerates metabolic rate of body
- increase cell metabolism, growth, differentiation, development
- increases protein, carbohydrate and fat metabolism
what do thyroid hormones influence
metabolism and function of all organs
how do thyroid hormones function
via nuclear receptors
what do thyroid hormone receptors do
regulate gene expression
how are thyroid hormone receptor functions defined
mutations that cause resistance to thyroid hormone: RTH
* mutations in TR beta and alpha
what does UCP1 do
increases the conductance of the inner mitochondrial membrane to make BAT mitochondria generate heat rather than ATP
what does stress do to the HPT axis
shuts it down - net lower TH production
what does cold do to the HPT axis
activates expression of thyroid hormone
how does T4 regulate cancer
via the plasma membrane associated integrin receptors
describe hypothyroidism
- lethargy and obestity (most common)
- derm symptoms
- alopcecia along trunk or tail head
- skin thickened
- dull dry hair
what are less common signs of hypothyroidism in dogs
- cardiovascular
- neuromuscular myopathies and megaesophagus
- facial nerve paralysis, vestibular disease, lower motor neuron disorders
- repro disorders in females
- corneal lipid reposits and GI problems
how to treat canine hypothyroidism
levothyroxine = synthetic T4
what is the most common type of primary hypothyroidism in dogs
lymphocytic thyroiditis
what is lymphocytic thyroiditis
- circulating thyroid autoantibodies
- antibodies to follicular cell, colloid, or thyroglobulin antigens
what do high doses of glucocorticoids do to thyroid function
reduce conversion of thyroxine (T4) to the more active triiodothyronine (T3)
hyperthyroidism
most common endocrinopathy of cats, typically caused by adenomatous hyperplasia of the thyroid gland
what are clinical signs of hyperthyroidism
- hypermetabolism
- activation of symathetic nervous system
- long-standing hyperthyroidism leads to hypertrophic cardiomyopathy, and death
- liver hypermetabolism
what is toxic nodular goiter caused by
aberrant signal transduction of the thyroid follicular cells
what does the TSH receptor activate
hyperthyroidism cats
receptor-coupled guanosine triphosphate-binding proteins (G proteins) in thyroid follicular cells
does diet play a role in feline hyperthyroidism
canned cat food has been implicated as a cause because BADGE, substance used to make liner of easy-open cans and cats susceptible to toxic effects of BADGE
what is the treatment of choice for cats with hyperthyroidism
radioactive iodine (RAI) therapy
what does felimazole (methimazole) do
prevents iodine and perozidase form their normal interactions with thyroglobulin to form T4 and T3
what is RAI therapy
- thyroid gland absorbs all iodine in blood
- large dose of RAI is taken, the radiation collects in thyroid cells
- destroys the thyroid gland and cancer cells
what are the advantages and disadvantages to RAI therapy
advantages
* gets tid of tumor completely
* targets thyroid tumor cells elsewhere in body
disadvantages
* requires cat to be in tx ward for 2 wks
* kitty litter is handles as radioactive waste
