Thyroid Gland Physiology

Question 1

where is the thyroid gland located

Answer 1

on either side of the trachea

NO isthmus in most veterinary species

Question 2

what embryonic layer does the thyroid develop from

Answer 2

endoderm

remnant/ectopic thyroid tissue along the neck is common in cats

Question 3

thyroid follicles

Answer 3

spherical structures surrounded by follicular epithelium and filled with colloid

Question 4

follicular cells

Answer 4

secrete thyroglobulin (proteinaceous) into the lumen of the follicle

Question 5

what is colloid made of

Answer 5

storage site for thyroglobulin + hormone precursors + iodide

Question 6

parafollicular cells

Answer 6

located between follicles

secretes calcitonin

Question 7

TRH

Answer 7

thyroid releasing hormone

produced in the hypothalamus to stimulate TSH secretion from the pituitary

Question 8

TSH

Answer 8

thyroid stimulating hormone

produced in the pituitary to stimulate T3/T4 production from the thyroid

Question 9

Tg

Answer 9

thyroglobulin

produced by the follicular cells as a building block for thyroid hormones

Question 10

T3

Answer 10

tri-iodothyronin

produced in the thyroid and in the peripheral tissues from conversion from T4

Question 11

rT3

Answer 11

reverse T3

produced primarily by conversion from T4 in peripheral tissues (some in thyroid); inactive form of T3

Question 12

T4

Answer 12

thyroxine

produced only in the thyroid

Question 13

fT4

Answer 13

free T4

produced in the thyroid; biologically active version of T4

Question 14

TGAA

Answer 14

thyroglobulin auto-antibodies; marker of immune mediated thyroiditis

Question 15

function of thyroid hormones

Answer 15

increase metabolism in all tissues

necessary for fetal development

temperature regulation, nutrient metabolism, hormone secretion, hair growth, erythropoiesis

Question 16

what receptor type do thyroid hormones use

Answer 16

nuclear receptors (increase gene transcription)

Question 17

iodide trapping

Answer 17

uptake of iodide from the GI tract into the follicular cells

depends on TSH concentration

Question 18

steps of thyroid hormone synthesis

Answer 18

1. ER of follicular cells produces and secretes thyroglobulin
2. thyroglobulin gets exocytosed out of the cell into the colloid lumen
3. iodide gets absorbed from the bloodstream into the follicular cell via Na/I symporter, then shuttled across the cell and into the colloid
4. thyroid peroxidase uses iodide to iodinate the tyrosine residues on thyroglobulin
5. tyrosine residues on thyroglobulin get conjugated together to form T3/T4, then the whole molecule gets endocytosed back into the follicular cell
6. T3/4 gets cleaved off of the thyroglobulin via proteolysis and exit out of the cell into the bloodstream

Question 19

how does the body eliminate thyroid hormone

Answer 19

inactivated by conjugation in the liver and excreted in bile

T3: sulfated
T4: glucuronidated

Question 20

HPT axis

Answer 20

hypothalamic-pituitary-thyroid axis

TRH –> TSH –> T3/T4

Question 21

what form is the majority of thyroid hormone produced by the thyroid

Answer 21

free T4

Question 22

what form is the majority of thyroid hormone in the body (circulation)

Answer 22

protein bound T4

Question 23

how much of total T3 in the body comes from thyroid production vs conversion in peripheral tissues

Answer 23

50% produced in thyroid
50% converted in peripheral tissues

Question 24

hypothyroidism

Answer 24

low levels of thyroid hormone

usually primary NOT secondary (pituitary)

Question 25

what species is hypothyroidism most common in

Answer 25

adult dogs, young kittens

Question 26

clinical signs of hypothyroidism

Answer 26

signs of low metabolism
- weight gain
- lethargy
- endocrine alopecia

Question 27

lab values with hypothyroidism

Answer 27

• NNN anemia
• hypercholesterolemia
• hypertriglyceridemia
• proteinuria

LOW T4 and fT4
HIGH TSH

Question 28

treatment for hypothyroidism

Answer 28

oral thyroxine

Question 29

hyperthyroidism

Answer 29

high levels of thyroid hormone; usually primary not secondary

Question 30

what species is hyperthyroidism most common in

Answer 30

old cats

Question 31

clinical signs of hyperthyroidism

Answer 31

signs of high metabolism
- weight loss
- polyphagia
- vomiting/diarrhea
- PU/PD
- hyperactivity
- thyroid slip
- thin/unkempt
- cardiac signs

Question 32

lab values of hyperthyroidism

Answer 32

• elevated liver enzymes
• isosthenuria
• polycythemia

HIGH T4 and fT4
LOW TSH

Question 33

treatment for hyperthyroidism

Answer 33

oral methimazole (inhibits thyroid peroxidase)

• low iodine diet
• thyroidectomy
• radiation

Question 34

euthyroid sick syndrome

Answer 34

decrease in thyroid hormone production due to high levels of cortisol during illness

protective mechanism - decreases metabolism during illness to promote healing and recovery

Question 35

lab values during illness

Answer 35

low T4
normal fT4 and TSH

Question 36

lab values during recovery

Answer 36

low T4
normal fT4
high TSH

Question 37

why should you not test an animal for hypothyroidism while it’s sick

Answer 37

because the values for ESS during recovery from illness and hypothyroidism are the same

low T4, high TSH