case 9 - teach me physiology Flashcards

1
Q

describe the anatomy of the thyroid gland

A

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.

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2
Q

what supplies the thyroid gland

A

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

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3
Q

what is the function of the thyroid gland

A

is to produce and store thyroid hromones.

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4
Q

what are thyroid epithelial that form follicles filled with

A

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

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5
Q

where can the parafollicular cells be found

A

in the spaces between the follicles

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6
Q

what do parafollicular cells do

A

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

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7
Q

what is the function of the thyroid

A

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

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8
Q

how do T3 and T4 act

A

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

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9
Q

what happens if there are high levels of them

A

they typically cause these processes to occur faster or more frequently

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10
Q

what metabolic processes increase by thyroid hormones

A

Basal Metabolic Rate
Gluconeogenesis
Glycogenolysis
Protein synthesis
Lipogenesis
Thermogenesis

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11
Q

how is this achieved

A

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

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12
Q

what is the pneumonic to remember thyroid hormone synthesis

A

ATE ICE

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13
Q

what are the six steps

A

active transport
thyroglobulin
exocytosis
iodination
coupling
endocytosis

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14
Q

what is the active transport step

A

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.

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15
Q

what is the thyroglobulin step

A

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

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16
Q

what is the exocytosis step

A

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

17
Q

what is the iodination step

A

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).

18
Q

what is the coupling step

A

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.

19
Q

what is the endocytosis step

A

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.

20
Q

what are the active thyroid hormones

A

T3 and T4

21
Q

what are some features of t2 and t4

A

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

22
Q

what is the differences between t3 and t4

A

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.

23
Q

how is T4 turned into T3

A

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

24
Q

how are T3 and T4 deactivated

A

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

25
Q

when are thyroid hormones released

A

as part of the HPT axis

26
Q

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

A

releases TRH into the hypophyseal portal system

27
Q

what does TRH bind to

A

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

28
Q

what happens to this TSH

A

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

29
Q

what is goitre

A

an enlarged thyroid gland

30
Q

what is hyperthyroidism

A

overactive thyroid gland

31
Q

what is Graves disease

A

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

32
Q

what else may happen to patients with hyperthyroidism

A

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

33
Q

how is hyperthyroidism treated

A

with carbimazole which inhibits iodine binding to thyroglobulin

34
Q

what is hypothyroidism

A

an underactive thyroid.

35
Q

what is the most common cause of hypothyroidism

A

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

36
Q

what is the most common cause of hypothyroidism

A

iodine deficinecy

37
Q

what can patients experience when having hypothyroidism

A

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

38
Q

what can hypothyroidism be treated

A

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