Thyroid Hormone

Question 1

where is the thyroid gland found?

Answer 1

either side of the trachea, just below the larynx
not normally palpable in cats and dogs

Question 2

thyroid gland composed of?

Answer 2

of follicles divided into clusters by connective tissue

Question 3

follicles are filled with?

Answer 3

filled with colloid (which stains pink due to its proteinaceous nature), the main storage site for thyroid hormones bound to thryoglobulin

Question 4

primary physiological function of thyroid hormone?

Answer 4

normal development, physical and mental well-being
growth
metabolism (increases O2 consumption & thermogenesis)
normal development of CNS
cardiovascular stimulant
promotes target cell responsiveness to sympathetic nervous system
Normal gonadal function

Question 5

biochemical actions of thyroid gland?

Answer 5

thermogenesis
carb metabolism
lipid metabolism
protein metabolism

Question 6

what are the steps in the synthesis of thyroid hormone?

Answer 6

1) follicular cells produce thyroglobulin which is sent into the colloid
2) thyroid stimulating hormone (TSH) increases iodide uptake
3) Iodination of thyroglobulin occurs which produces Thyroxine (T4) and Triiodothyronine (T3) they are still attached to thyroglobulin atp

Question 7

active follicle of thyroid gland?

Answer 7

stored thyroid hormones are being extracted from the lumen and converted into active thyroid hormones

Question 8

describe the epithelial cells of an inactive follicle?

Answer 8

epithelial cells relatively flattened

Question 9

describe the epithelial cells of an active follicle?

Answer 9

epithelial cells taller and notice white edge on histological slide - due to non staining vacuoles, called reabsorption lacunae

Question 10

levels of TH when hypothyroidism is occurring?

Answer 10

suboptimal levels of TH

Question 11

Clinical signs of hypothyroidism?

Answer 11

reduction in overall metabolic rate:
heat seeking
mental retardation, dullness, sluggish, fatigue
slow reflexes + response
weight gain
weak pulse, bradycardia
SKIN ISSUES:
bilateral symmetrical alopecia (non-pruritic)
skin thickening

Question 12

clinical signs of hyperthyroidism?

Answer 12

increased body temp
polydipsia (thirst) and polyuria (incr urination)
weight loss despite appetite
elevated heart rate and workload
increased metabolic rate leading to loss of skeletal mass

Question 13

what can untreated hyperthyroid cats develop?

Answer 13

can develop heart failure

Question 14

what is a parenchyma?

Answer 14

it is the tissue of an animal organ (as a gland) which performs the work of the organ instead of just being supportive tissue

Question 15

what is the role of connective tissue in the thyroid gland?

Answer 15

it supports the parenchyma and carries blood vessels, lymphatics and nerves

Question 16

the thyroid gland is composed of follicles, describe these follicles:

Answer 16

each follicle is a sphere of varying size with a wall of a single layer of epithelial cells (follicular cells) involved in thyroid hormone synthesis and secretion

Question 17

what are the follicles of the thyroid gland filled with?

Answer 17

filled with colloid - the main storage site for thyroid hormones bound to thyroglobulin

Question 18

what colour does colloid stain?

Answer 18

it stains pink due to proteinaceous nature

Question 19

what are C-cells and where are these located?

Answer 19

C-cells are located in the connective tissue between follicles and produce calcitonin (involved in calcium regulation)

Question 20

describe the follicle wall of the follicles of the thyroid gland:

Answer 20

follicle wall is a single layer of cuboidal epithelial cells; lumen is filled with colloid

Question 21

how is the thyroid hormone essential for normal development?

Answer 21

it is essential for normal development of body tissue and for physical and mental well-being
every tissue is affected (in)directly by thyroid hormone

Question 22

how does the thyroid hormone affect growth?

Answer 22

thyroid stimulates growth hormone secretion and increases production of IGF-1 by the liver, promoting the synthesis of structural proteins and skeletal growth

Question 23

what generalised effects does the thyroid hormone have?

Answer 23

activates nuclear transcription of large numbers of genes in virtually all cells of the body.
Allows for enzymes, structural and transport proteins to be synthesised, leading to an increase in functional activity throughout the body

Question 24

how is the thyroid hormones involved in metabolism?

Answer 24

increases overall basal metabolic rate
regulates the rate of O2 consumption and energy expenditure at rest
increased metabolic activity leads to increased heat production
Involved in both synthesis and degradation of fat, carb and protein

Question 25

how is the thyroid hormone involved in normal development of the nervous system?

Answer 25

adequate production of thyroid hormone is important during foetal life & early brain development. Involved in myelination, thus impacting axonal speed conduction

Question 26

how does the thyroid hormone affect the cardiovascular system?

Answer 26

it increases blood flow and cardiac output and increases heart rate (related to thyroid hormone promoting target cell responsiveness to the sympathetic nervous system) increased heat production also leads to peripheral vasodilation

Question 27

how does the thyroid hormone help with normal gonadal functions?

Answer 27

important for the function of the testes and ovaries - involved in sperm production and the oestrus cycle

Question 28

what is thermogenesis?

Answer 28

it increases heat production and oxygen consumption by increasing mitochondrial oxidative phosphorylation

Question 29

what effect does the thyroid hormone have on carbohydrate metabolism?

Answer 29

it stimulates glycogenolysis in the liver and potentiates the effect of adrenaline enhances intestinal glucose absorption

Question 30

what effect does the thyroid hormone have on lipid metabolism?

Answer 30

stimulates the release of free fatty acids by increasing the activity of hormone-sensitive lipase in adipocytes stimulates the oxidation of fatty acids enhances hepatic conversion of cholesterol to bile acids

Question 31

what effect does thyroid hormone have on protein metabolism?

Answer 31

stimulates the synthesis of proteins mainly involved in oxidative reactions stimulates protein catabolism

Question 32

what is protein catabolism?

Answer 32

it is the breakdown of proteins into absorbable monomers for further degradation or reassembly

Question 33

how is the thyroid hormone regulated?

Answer 33

via the hypothalamic-pituitary-thyroid axis

Question 34

where is the TRH produced and what does it do?

Answer 34

it is produced in the hypothalamus and it stimulates the anterior pituitary gland

Question 35

what is TRH?

Answer 35

Thyrotropin-releasing hormone

Question 36

What is TSH?

Answer 36

thyroid stimulating hormone

Question 37

what produces the TSH?

Answer 37

produced by the anterior pituitary gland

Question 38

What requires TSH? (thyroid stimulating hormone)

Answer 38

the release of thyroid hormone

Question 39

what does TSH do?

Answer 39

it binds to membrane receptors of thyroid cells required for thyroid hormone release (no TSH = no thyroid hormone) regulated the no. and size of follicular cells

Question 40

describe the negative feedback for regulating the thyroid hormone:

Answer 40

circulating free T3 and T4 hormones inhibit the hypothalamus and anterior pituitary

Question 41

what temp promotes the release of thyroid hormone?

Answer 41

low temps

Question 42

collectively, what are T3 and T4 hormone known as?

Answer 42

known as thyroid hormone

Question 43

where do all steps involved in the synthesis of thyroid hormone take place?

Answer 43

they take place in the colloid and all things needed for the synthesis are also found in the colloid

Question 45

how do follicular cells take up colloid?

Answer 45

by endocytosis

Question 46

where do T3 and T4 need to be released from?

Answer 46

needs to be released from thyroglobulin

Question 47

what is T4?

Answer 47

thyroxine

Question 48

what is T3?

Answer 48

triiodothyronine

Question 49

where is colloid contained?

Answer 49

it is contained in endosome - fuses with lysosome which contains enzyme which releases T3 and T4 from thyroglobulin

Question 50

T3 and T4 are?

Answer 50

lipid soluble

Question 51

if T3 and T4 are lipid soluble, what does this mean they can do?

Answer 51

they can diffuse through basal membrane of follicular cell into blood

Question 52

where do thyroid hormones bind?

Answer 52

they bind to carrier proteins in blood for transport to target cells

Question 53

free T3 and T4 are...

Answer 53

biologically active

Question 54

what happens when a follicle is active?

Answer 54

stored thyroid hormones are being extracted from the lumen and converted into active thyroid hormones

Question 55

what happens during the circulation of thyroid hormone?

Answer 55

mostly T4 secreted, some T3 T3 is more biologically active Bulk of T3 is derived by deiodination of T4 in peripheral tissues

Question 56

99.5% of circulating thyroid hormone is bound to?

Answer 56

bound to plasma proteins

Question 57

what is the main carrier protein for transporting thyroid hormone?

Answer 57

thyroxine binding globulin (TBG)

Question 58

Apart from TBG, what else does thyroid hormone bind to?

Answer 58

some also binds to thyroxine binding pre albumin (TBPA) and albumin

Question 59

what is TBG?

Answer 59

thyroxine binding globulin

Question 60

TBPA?

Answer 60

Thyroxine binding pre albumin

Question 61

what are the two types of hypothyroidism?

Answer 61

primary and secondary

Question 62

describe the primary type hypothyroidism:

Answer 62

related to the thyroid gland itself (most common) plasma TSH is high

Question 63

what does primary hypothyroidism cause?

Answer 63

lymphocytic thyroiditis, Idiopathic atrophy of the thyroid gland

Question 64

describe the secondary type of hypothyroidism?

Answer 64

related to the pituitary gland/hypothalamus and plasma TSH is low

Question 65

what are the results of hyperthyroidism?

Answer 65

excessive production of thyroid hormones (T3 and T4) Leads to an increase in overall metabolic rate Commonly seen in CATS Typically associated w/ benign thyroid tumours that increase hormone production

Question 66

Goitre in animals is what?

Answer 66

enlarged thyroid gland

Question 67

what are the common causes of goitre in animals?

Answer 67

Idiopathic Iodine deficiency Iodine toxicity Ingestion of goitrogenic plants

Question 68

what is a goitre?

Answer 68

a lump of swelling at the front of the neck