Thyroid Hormones

Question 1

What is thyroglobulin?

Answer 1

protein made by ER of follicular cells and exocytosed into colloid. Its tyrosyl residues are precursors of thyroid hormones

Question 2

What are the thyroid hormones?

Answer 2

T3 (triiodothyronine) and T4 (thyroxine)

Question 3

Which thyroid hormone is more potent?

Answer 3

T3

Question 4

What is rT3?

Answer 4

reverse T3 (inactive form)

Question 5

How are thyroid hormones synthesized in thyroid gland?

Answer 5

1. Iodide is taken up by sodium-iodide symporter in basolateral membrane then extruded on apical side by pendrin
2. iodide is oxidized in colloid by thyroid peroxidase
3. tyrosyl residues in thyroglobulin is iodinated by thyroid peroxidase
4. MIT (mono-iodotyrosil) and DIT (di-iodotyrosil) residues are formed
5. MIT and DIT are conjugated to T3 or T4 by thryoid peroxidase
6. thyroglublin with T3, T4 are taken up from lumen of follicle cell by endocytosis
7. vesicles fuse with lysosomes
8. proteolysis leads to release of T3 and T4
9. T3 and T4 are secreted into circulation

Question 6

What can inhibit sodium-iodide symporter?

Answer 6

complex anions

Question 7

Where are thyroid hormones produced?

Answer 7

Thyroid gland: directly produces T3 and T4
Peripheral tissues: produces most of the T3 by metabolizing T4

Question 8

What metabolizes T4?

Answer 8

Type I deiodinase: peripheral tissues
Type II deiodinase: brain and brown adipose tissue
Type III deiodinase: liver and CNS (it inactivates T4 and T3)

Question 9

What is TSH?

Answer 9

thyroid stimulating hormone - it stimulates all steps in the thyroid hormone synthetic pathway and growth of thyroid gland. It is secreted by anterior pituitary

Question 10

What is TRH?

Answer 10

thyrotropin releasing hormone - stimulates TSH secretion. Secreted by hypothalamus

Question 11

____________ exerts negative feedback effects on TRH and TSH secretion

Answer 11

thyroid hormones (T3 & T4)

Question 12

How does TSH stimulate thyroid hormone synthesis?

Answer 12

stimulates all steps in thyroid hormone synthesis, like iodination of tyrosyl groups

Question 13

How does TSH stimulate thyroid hormone secretion?

Answer 13

it causes the secretion of pre-stored thyroid hormones by stimulating all steps like proteolysis of thyroglobulin and secretion of thyroid hormones

Question 14

How does TSH cause growth of thyroid gland?

Answer 14

it increases hypertrophy & hyperplasia of thyroid follicular cells

Question 15

How are thyroid hormones transported in circulation?

Answer 15

1. thyroxine-binding globulin (TBG) - very high affinity
2. thyroxine-binding prealbumin (TBPA)
3. albumin
4. free fraction

Question 16

What is the advantage of transport proteins for thyroid hormones?

Answer 16

It carries the bulk of thyroid hormone in circulation and can act as a reservoir

Question 17

About ____% of plasma T4 is free in cats and dogs

Answer 17

0.1% (less in humans)

Question 18

About ___% of plasma T3 is free in cats and dogs

Answer 18

1% (less in humans)

Question 19

How does the rate of thyroid hormone degradation compare in animals vs humans?

Answer 19

Degradation is faster in animals because
1. weak transport protein binding leads to lower total T4 concentration (smaller reservoir)
2. higher free T4 is available for degradation

Question 20

How fast is circulating thyroid hormones replaced in dogs and cats?

Answer 20

T3: within half a day
T4: within one day

Question 21

How are thyroid hormones degraded?

Answer 21

They are degraded in the liver by conjugation with glucoronic and sulfuric acids and then excreted in bile. Once in SI, thyroid hormones are hydrolyzed (de-conjugated) and then reabsorbed into portal vein (enterohepatic circulation) or eliminated in feces.

Question 22

What are the genomic actions of thyroid hormones?

Answer 22

Thyroid hormones, primarily T3, bind nuclear thyroid hormones which has many different isoforms (TRα1, TRα2, TRβ1, TRβ2) that are tissue specific.

Question 23

What are the effects of thyroid hormones on body metabolism?

Answer 23

1. regulate synthesis of proteins involved in metabolism
2. increase oxygen consumption and heat generation by mitochondria

Question 24

What the the effects of thyroid hormones on basal metabolism?

Answer 24

regulates BMR by stimulating expression of thermogenin and its response to adrenergic stimulation (sympathetic NS)

Question 25

What is thermogenin?

Answer 25

an uncoupling protein (UCP) that allows the release of energy as heat instead of ATP formation

Question 26

What is BMR?

Answer 26

basal metabolic rate - rate of energy generation and use at complete rest

Question 27

What releases more heat? white adipose tissue or brown adipose tissue?

Answer 27

brown

Question 28

What are the effects of thyroid hormones on fetal brain development?

Answer 28

It is required in the early days/year of life for maturation of brain and CNS. If they are deficient they can become mentally retarded

Question 29

What are the effects of thyroid hormones on growth?

Answer 29

thyroid hormones indirectly influence growth of different organs because growth hormone secretion/functions require them

Question 30

What are the effects of thyroid hormone on cardiovascular system?

Answer 30

1. increase cardiac contractility 2. increase oxygen demand of heart muscle

Question 31

What is hypothyroidism?

Answer 31

A spontaneous condition in dogs caused by autoimmune destruction of thyroid gland, iodine deficiency, or consumption of goitrogen

Question 32

What is goitrogen?

Answer 32

suppressor of thyroid hormone synthesis

Question 33

What are the signs of hypothyroidism?

Answer 33

1. cold intolerance 2. decreased heart parameters 3. alopecia 4. increased TSH levels

Question 34

What is hyperthyroidism?

Answer 34

a spontaneous condition in cats that is caused by autonomous hyperfunction of thyroid gland

Question 35

What are the signs of hyperthyroidism?

Answer 35

1. increased heart parameters 2. nervousness, hyperactivity 3. low grade intermittent fever 4. matted coat

Question 36

What is goiter?

Answer 36

enlargement of thyroid gland because of either hypothyroidism (iodine deficiency) or hyperthyroidism

Question 37

How can you prevent iodine-deficient goiter?

Answer 37

iodized salt

Question 38

What is thermoregulation?

Answer 38

the control of core (deep tissue) body temperature

Question 39

What are endotherms?

Answer 39

organism that can control their internal body temperature by generating heat to raise temp or using various processes to lose heat.

Question 40

What are ectotherms?

Answer 40

organisms that cannot control their internal body temperature and instead gain most of body heat from surroundings

Question 41

What are the modes of heat gain/loss?

Answer 41

1. conduction (contact between objects) 2. convection (contact with fluids in contact with skin) 3. radiation (thermal energy from visible or non-visible infrared regions of electromagnetic spectrum) 4. evaporation (fluid absorbs heat from body and body is cooled)

Question 42

What are thermoreceptors?

Answer 42

receptive ends of sensory neurons which detect changes in temperature via cold and warm receptors they are located in the skin, hypothalamus, etc (though the skin ones are most sensitive)

Question 43

T/F: thermoreceptors detect pain caused by temperature

Answer 43

false: pain is detected by nocireceptors

Question 44

What part of the brain controls thermoregulation?

Answer 44

Thermoregulatory areas of hypothalamus: heat loss area (anterior) and heat gain area (posterior)

Question 45

What are the corrective mechanisms used in a cold environment?

Answer 45

1. vasoconstriction of skin vessels 2. piloerection/pilomotion/ptilomotion 3. shivering thermogenesis 4. decreased sweating 5. increased metabolism

Question 46

What is piloerection?

Answer 46

Elevation of fur caused by arrector pili muscle contraction. It causes air to be trapped and increases insulation. Also known as pilomotion (different than ptilomotion which is for feathers)

Question 47

What are the corrective mechanisms used in hot environments?

Answer 47

1. vasodilation of skin vessels 2. increased sweating 3. panting 4. pilorelaxation 5. decreased metabolism 6. behavioral change (ie. shade seeking)

Question 48

How does panting help?

Answer 48

Water is lost via evaporation from moist surfaces. Mucosa cools down and venous drainage carries cooled blood to heart and body.

Question 49

What is gular fluttering?

Answer 49

It is when birds use rapid movement of gular (throat) skin to lose heat by evaporation and convection (due to rapid movement of air).

Question 50

What are modes of evaporative heat loss in mammals and birds?

Answer 50

1. panting 2. gular fluttering 3. sweating 4. saliva spreading

Question 51

What is fever?

Answer 51

well-controlled state of elevated temperature set-point by hypothalamus

Question 52

What is hyperthermia?

Answer 52

heat gain due to failure of thermoregulatory process

Question 53

What are pyrogens?

Answer 53

molecules that enhance the activities of cold-receptors while suppressing the activities of warm receptors, ultimately inducing fever. They do this by stimulating microglia to produce prostaglandin which acts on neurosensitive neurons in hypothalamus. They can be endogenous (from immune system) or exogenous (from microorganisms)