Thyroid Gland Physiology Flashcards
where is the thyroid gland located
on either side of the trachea
NO isthmus in most veterinary species
what embryonic layer does the thyroid develop from
endoderm
remnant/ectopic thyroid tissue along the neck is common in cats
thyroid follicles
spherical structures surrounded by follicular epithelium and filled with colloid
follicular cells
secrete thyroglobulin (proteinaceous) into the lumen of the follicle
what is colloid made of
storage site for thyroglobulin + hormone precursors + iodide
parafollicular cells
located between follicles
secretes calcitonin
TRH
thyroid releasing hormone
produced in the hypothalamus to stimulate TSH secretion from the pituitary
TSH
thyroid stimulating hormone
produced in the pituitary to stimulate T3/T4 production from the thyroid
Tg
thyroglobulin
produced by the follicular cells as a building block for thyroid hormones
T3
tri-iodothyronin
produced in the thyroid and in the peripheral tissues from conversion from T4
rT3
reverse T3
produced primarily by conversion from T4 in peripheral tissues (some in thyroid); inactive form of T3
T4
thyroxine
produced only in the thyroid
fT4
free T4
produced in the thyroid; biologically active version of T4
TGAA
thyroglobulin auto-antibodies; marker of immune mediated thyroiditis
function of thyroid hormones
increase metabolism in all tissues
necessary for fetal development
temperature regulation, nutrient metabolism, hormone secretion, hair growth, erythropoiesis
what receptor type do thyroid hormones use
nuclear receptors (increase gene transcription)
iodide trapping
uptake of iodide from the GI tract into the follicular cells
depends on TSH concentration
steps of thyroid hormone synthesis
- ER of follicular cells produces and secretes thyroglobulin
- thyroglobulin gets exocytosed out of the cell into the colloid lumen
- iodide gets absorbed from the bloodstream into the follicular cell via Na/I symporter, then shuttled across the cell and into the colloid
- thyroid peroxidase uses iodide to iodinate the tyrosine residues on thyroglobulin
- tyrosine residues on thyroglobulin get conjugated together to form T3/T4, then the whole molecule gets endocytosed back into the follicular cell
- T3/4 gets cleaved off of the thyroglobulin via proteolysis and exit out of the cell into the bloodstream
how does the body eliminate thyroid hormone
inactivated by conjugation in the liver and excreted in bile
T3: sulfated
T4: glucuronidated
HPT axis
hypothalamic-pituitary-thyroid axis
TRH –> TSH –> T3/T4
what form is the majority of thyroid hormone produced by the thyroid
free T4
what form is the majority of thyroid hormone in the body (circulation)
protein bound T4
how much of total T3 in the body comes from thyroid production vs conversion in peripheral tissues
50% produced in thyroid
50% converted in peripheral tissues
hypothyroidism
low levels of thyroid hormone
usually primary NOT secondary (pituitary)
what species is hypothyroidism most common in
adult dogs, young kittens
clinical signs of hypothyroidism
signs of low metabolism
- weight gain
- lethargy
- endocrine alopecia
lab values with hypothyroidism
- NNN anemia
- hypercholesterolemia
- hypertriglyceridemia
- proteinuria
LOW T4 and fT4
HIGH TSH
treatment for hypothyroidism
oral thyroxine
hyperthyroidism
high levels of thyroid hormone; usually primary not secondary
what species is hyperthyroidism most common in
old cats
clinical signs of hyperthyroidism
signs of high metabolism
- weight loss
- polyphagia
- vomiting/diarrhea
- PU/PD
- hyperactivity
- thyroid slip
- thin/unkempt
- cardiac signs
lab values of hyperthyroidism
- elevated liver enzymes
- isosthenuria
- polycythemia
HIGH T4 and fT4
LOW TSH
treatment for hyperthyroidism
oral methimazole (inhibits thyroid peroxidase)
- low iodine diet
- thyroidectomy
- radiation
euthyroid sick syndrome
decrease in thyroid hormone production due to high levels of cortisol during illness
protective mechanism - decreases metabolism during illness to promote healing and recovery
lab values during illness
low T4
normal fT4 and TSH
lab values during recovery
low T4
normal fT4
high TSH
why should you not test an animal for hypothyroidism while it’s sick
because the values for ESS during recovery from illness and hypothyroidism are the same
low T4, high TSH
