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Endocrine > The Thyroid Gland > Flashcards

The Thyroid Gland Flashcards

1
Q

Where does the thyroid gland lie?

A

Across the base of the trachea

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2
Q

What is the main hormones produced by the thyroid gland?

A

T3 (triiodothyronine) and T4 (thyroxine)

The thyroid hormones

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3
Q

What kinds of cells does the thyroid gland contain?

A

C (clear) cells

Follicular cells

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4
Q

What is the role of clear cells?

A

Produce calcitonin

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5
Q

What is the role of follicular cells?

A

Support thyroid hormone synthesis and surround hollow follicles

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6
Q

What are thyroid follicles?

A

Spherical structures whose falls are made of follicular cells
Centre is filled with colloid

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7
Q

What is colloid?

A

sticky glycoprotein matrix

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8
Q

What do follicular cells make?

A

Enzymes to make thyroid hormones + thyroglobulin

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9
Q

What happens to the thyroid enzymes and thyroglobulin?

A

Packaged into vesicles and exported from follicular cells into the colloid

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10
Q

Apart from making thyroglobulin + enzymes what else do the follicular cells do?

A

Actively concentrate iodide from the plasma + transport it into colloid where it combined with tyrosine residues to make thyroid hormones

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11
Q

Where are tyrosine and iodide derived from?

A

The diet

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12
Q

What is thyroglobulin?

A

A large protein rich in tyrosine residues

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13
Q

What is T4 composed of?

A

2 tyrosine + 4 iodide

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14
Q

What is T3 composed of?

A

2 tyrosine + 3 iodide

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15
Q

What enzyme catalyses the synthesise of thyroid hormones from the tyrosine residues?

A

Thyroid perioxidase (located on apical membrane of follicular cells)

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16
Q

How does iodide enter the follicular cells from the plasma?

A

Via a Na/I symport

coupling to Na enables follicular cells to take up iodide against a conc. gradient

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17
Q

How is iodide then transported to the colloid?

A

Via the pendrin transporter

18
Q

What inhibits iodide transport into the thyroid gland?

A

Thiocynates, compounds formed from detoxification of cyanide (common origin is cigarette smoke)

19
Q

Enzymes are exocytosed into the colloid along with thyroglobulin catalyse what reaction?

A

Addition of iodide to tyrosine residues
Addition of one iodine to trysoine –> monoiodotryosine (MIT)
Addition of a second iodine –> Diiodotryosine)

MIT + DIT –> T3
DIT + DIT –> T4

20
Q

What does TSH stimulate in the thyroid follicles?

A

For portions of colloid to be taken into the follicular cell by endocytosis
Within the cells they form vesicles which contain proteolytic enzymes that cut thyroglobulin to release the thyroid hormones

21
Q

What happens after the thyroid hormones are released from thyroglobulin?

A

T3/T4 are lipid soluble so cross follicular membrane into plasma + bind to plasma proteins, mainly thyroxine binding globulin and then circulate in the plasma

22
Q

What % of T3/T4 is free in the plasma?

A

0.2% - very highly protein bound

23
Q

Why does T 4 have a higher half life than T3?

A

Thyroxine binding globulin has a particularly high affinity for T4 so releases it slowly into the plasma

24
Q

What hormone can exert an inhibitor effect on TSH and TRH?

A

Only free hormone

25
Q

Which of T3 and T4 is more abundant in the plasma?

A

T4

26
Q

Which of T3 and T4 binds to TH receptors inside cells more?

A

T3 - TH receptor has much higher affinity for T3 so it is more physiologically active than T4

27
Q

Around half the T4 in plasma is what?

A

Deiodinated (with the remaining fraction being deiodinated inside target cels)

This is the process by which T4 is transformed into T3 y deiodinase enzymes

28
Q

What things increase TRH release?

A

Cold, stress, pregnancy

29
Q

What things reduce thyroid hormone levels?

A

Glucocorticoids inhibit TSH and conversion of T4 to T3

Somatostatin inhibits TSH

30
Q

How do thyroid hormones exert their action?

A

Bind to nuclear receptors inside target cells where they alter transcription and translation to alter protein synthesis

31
Q

What are the roles of thyroid hormones?

A

Raises metabolic rate, promotes thermogenesis
Increases hepatic gluconeogenesis
Net increase in proteolysis
Net increase in lipolysis
Critical for growth –> anabolic, stimulates GH receptor expression
Required for foetal brain development

32
Q

Give two examples of things that may cause hyperthyroidism

A

Grave’s disease

Thyroid adenoma

33
Q

What is grave’s disease?

A

Antibodies produced that mimic TSH and continually activate thyroid gland
High TH, Low TSH
Thyroid gland may be 2-3x larger

34
Q

What is a thyroid adenoma?

A

Rare hormone secreting thyroid tumour

35
Q

What are the symptoms of hyperthyroidism?

A

Increased metabolic rate + heat production –> wt loss, heat intolerance

Increased protein catabolism –> muscle weakness, wt loss

Altered nervous system function –> hyperreflexia, psychological disturbances

Elevated CV function, TH permissive to epinephrine, B receptors –> increased HR/contractile force, high output, cardiac failure

36
Q

Give examples of things that cause hypothyroidism

A

Hashimoto’s
Iodine deficiency
Idiopathic, maybe linked to thyroiditis

37
Q

What is Hashimoto’s disease?

A

Autoimmune attack of thyroid gland

38
Q

What are good sources of iodine?

A

Fish, seafood, seaweed

39
Q

What are the symptoms of hypothyroidism?

A

Decreased metabolic rate + heat production –> wt gain, cold intolerance

Disrupted protein synthesis –> brittle nails/thin skin

Altered nervous system –> slow speech/reflexes, fatigue

Reduced CV function –> slow HR, weak pulse

40
Q

What is a goitre?

A

Enlargement of the thyroid gland

41
Q

What things may cause a goitre?

A

Increased trophic action of TSH on follicular cells (hypothyroidism)

Over-activity due to autoimmune disease (Grave’s)

Endocrine (11 decks)

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