case 9 - teach me physiology

Question 1

describe the anatomy of the thyroid gland

Answer 1

it is a ductless alveolar gland found in the anterior neck, just below the laryngeal prominence (Adam’s Apple). it is roughly butterfly shaped with two lobes wrapping around the trachea and connected in the middle by an isthmus. the thyroid gland is not usually palpable.

Question 2

what supplies the thyroid gland

Answer 2

it is supplied by the superior and inferior thyroid arteries, drained via superior, middle and inferior thyroid veins, and has a rich lymphatic system

Question 3

what is the function of the thyroid gland

Answer 3

is to produce and store thyroid hromones.

Question 4

what are thyroid epithelial that form follicles filled with

Answer 4

colloid - a protein-rick reservoir of the materials needed for thyroid hormone production. these follicles range in size and the epithelium may be simple or columnar

Question 5

where can the parafollicular cells be found

Answer 5

in the spaces between the follicles

Question 6

what do parafollicular cells do

Answer 6

secrete calcitonin, which is involved in the regulation of calcium metabolism in the body

Question 7

what is the function of the thyroid

Answer 7

the thyroid gland is one of the main regulators of metabolism.

Question 8

how do T3 and T4 act

Answer 8

typically act via nuclear receptors in target tissues and initiate a variety of metabolic pathways.

Question 9

what happens if there are high levels of them

Answer 9

they typically cause these processes to occur faster or more frequently

Question 10

what metabolic processes increase by thyroid hormones

Answer 10

Basal Metabolic Rate
Gluconeogenesis
Glycogenolysis
Protein synthesis
Lipogenesis
Thermogenesis

Question 11

how is this achieved

Answer 11

increase number and size of the mitochondria within cells, increasing Na-K pump activity and increasing the presence of beta-adrenergic receptors in tissues such as cardiac muslce

Question 12

what is the pneumonic to remember thyroid hormone synthesis

Answer 12

ATE ICE

Question 13

what are the six steps

Answer 13

active transport
thyroglobulin
exocytosis
iodination
coupling
endocytosis

Question 14

what is the active transport step

Answer 14

Active transport of Iodide into the follicular cell via the Sodium-Iodide Symporter (NIS). This is actually secondary active transport, and the sodium gradient driving it is maintained by a Sodium-Potassium ATPase.

Question 15

what is the thyroglobulin step

Answer 15

Thyroglobulin (Tg), a large protein rich in Tyrosine, is formed in follicular ribosomes and placed into secretory vesicles.

Question 16

what is the exocytosis step

Answer 16

Exocytosis of Thyroglobulin into the follicle lumen, where it is stored as colloid. Thyroglobulin is the scaffold upon which thyroid hormone is synthesised.

Question 17

what is the iodination step

Answer 17

Iodination of the Thyroglobulin. Iodide is made reactive by the enzyme thyroid peroxidase. Iodide binds to the benzene ring on Tyrosine residues of Thyroglobulin, forming monoiodotyrosine (MIT) then diiodotyrosine (DIT).

Question 18

what is the coupling step

Answer 18

Coupling of MIT and DIT gives the Triiodothyronine (T3) hormone and coupling of DIT and DIT gives the Tetraiodothyronine (T4) hormone, also known as Thyroxine.

Question 19

what is the endocytosis step

Answer 19

Endocytosis of iodinated thyroglobulin back into the follicular cell. Thyroglobulin undergoes proteolysis in lysosomes to cleave the iodinated tyrosine residues from the larger protein. Free T3 or T4 is then released, and the Thyroglobulin scaffold is recycled.

Question 20

what are the active thyroid hormones

Answer 20

T3 and T4

Question 21

what are some features of t2 and t4

Answer 21

they are fat soluble and mostly carried by plasma proteins - thyronine binding globulin and albumin

Question 22

what is the differences between t3 and t4

Answer 22

T3 is much more potent, but it has a shorter half life due to its lower affinity for the bindiing proteins. less than 1% of T3 and T4 is unbound free hormone.

Question 23

how is T4 turned into T3

Answer 23

at the peripheries, T4 is deiodinated to the more active T3

Question 24

how are T3 and T4 deactivated

Answer 24

by removing iodine. this happens in the liver and kidney.

Question 25

when are thyroid hormones released

Answer 25

as part of the HPT axis

Question 26

what happens when the hypothalamus detects a low plasma concentration of thyroid hormone

Answer 26

releases TRH into the hypophyseal portal system

Question 27

what does TRH bind to

Answer 27

receptors found on thyrotrophic cells of the anterior pituitary gland, causing them to release TSH into the systemic circulation.

Question 28

what happens to this TSH

Answer 28

it binds to TSH receptors on the basolateral membrane of thyroid follicular cells and induces the synthesis and release of thyroid hormone

Question 29

what is goitre

Answer 29

an enlarged thyroid gland

Question 30

what is hyperthyroidism

Answer 30

overactive thyroid gland

Question 31

what is Graves disease

Answer 31

an autoimmune condition where antibodies are produced that stimulate the TSH receptors on follicular cells. it affects roughly 1% of the population and is ten times more common in women than in men

Question 32

what else may happen to patients with hyperthyroidism

Answer 32

heat intolerance, weight loss, tachycardia, nervousness, increased sweating, exophthalmos

Question 33

how is hyperthyroidism treated

Answer 33

with carbimazole which inhibits iodine binding to thyroglobulin

Question 34

what is hypothyroidism

Answer 34

an underactive thyroid.

Question 35

what is the most common cause of hypothyroidism

Answer 35

Hashimoto’s Disease - an autoimmune condition where thyroid follicles are destroyed or antibodies are produced that block the TSH receptor on follicle cells

Question 36

what is the most common cause of hypothyroidism

Answer 36

iodine deficinecy

Question 37

what can patients experience when having hypothyroidism

Answer 37

Patients can present with cold intolerance, weight gain, bradycardia, poor concentration, myxoedema, dry skin, some hair loss and constipation.

Question 38

what can hypothyroidism be treated

Answer 38

Hypothyroidism can be treated with oral T4 tablets (100-200 µg/day), to replace the hormone that is not being produced by the body.