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2ND YEAR SYSTEMS: Diabetes and Endocrinology > The thyroid gland > Flashcards

The thyroid gland Flashcards

1
Q

What are the 2 physiologically active forms of thyroid hormone

A

T3 (triiodothyronine)

T4 (thyroxine)

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2
Q

What are the 2 main cell types of the thyroid gland

A

C (clear cells)

Follicular cells

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3
Q

What do c cells secrete

A

Calcitonin (ca2+ regulating hormone)

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4
Q

Function of follicular cells

A

support thyroid hormone synthesis ie T3+T4

manufacture enzymes that make thyroid hormones as well as thyroglobulin

and they line the follicles

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5
Q

What is the l of the thyroid follicle filled with?

A

Colloid - a sticky glycoprotein matrix

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6
Q

What 2 things are packaged into vesicles and exported from the follicular cells into the colloid?

A

Enzymes and thyroglobulin

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7
Q

What happens to the enzymes and thyroglobulin that follicular cells produce?

A

They are packaged into vesicles and exported from the follicular cells into the colloid

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8
Q

How do follicular cells form thyroid hormones

A

They actively concentrate iodide from the plasma and transport it into the colloid where it combines with the tyrosine residues to form the thyroid hormones.

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9
Q

Where are tyrosine and iodide derived from

A

diet

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10
Q

How does iodide enter the follicular cells

A

via a Na+/I- transporter

The coupling to Na+ enables the follicular cells to take up iodide against a concentration gradient

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11
Q

How does iodide enter the colloid

A

Via the pendrin transporter

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12
Q

More specific description of how T3 and T4 are produced

A

enzymes exocytosed into the colloid and thyroglobulin catalyse the oxidation of iodide to iodine

iodine + tyrosine residues on the thyroglobulin

Addition of one iodine to tyrosine = MIT (monoiodotyrosine).

Adding a second iodine = DIT (diiodotyrosine).

So MIT+DIT = triiodothyronine (T3)

DIT+DIT = tetraiodothyronine (T4)

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13
Q

What catalyses the reactions of tyrosine into T3 or T4

A

thyroid peroxidase

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14
Q

What is the response to thyroid stimulating hormone in follicular cells

A

they take back up portions of the colloid by endocytosis

within the follicular cells they form vesicles which contain proteolytic enzymes that cut thyroglobulin to release thyroid hormones

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15
Q

Why can T3 and T4 pass across the follicular cell membrane?

A

they are lipid soluble

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16
Q

What happens once T3 and T4 are in the plasma?

A

they bind to plasma proteins mainly thyroxine-binding globulin (99.8%) (rare mutations in this protein cause major disruption to TH balance)

17
Q

Movement of thyroid hormones from the colloid to the plasma is under what influence

A

under the influence of thyroid stimulating hormone released from the pituitary

TSH stimulates the follicular cells to endocytose colloidal thyroglobulin

18
Q

What is one reason why T4 has a longer half life than T3

A

Thyroxine Binding Globulin (TBG) has a particularly high affinity for T4, releasing it only slowly into the plasma.

19
Q

Why is T3 3-5 times more physiologically active than T4?

A

The TH receptor has a much higher affinity for T3

90% of TH binding to TH receptors inside cells is T3

20
Q

What happens when T4 is deiodinated?

A

it becomes T3

around half the T4 is deiodinated in the plasma and the rest is deiodinated inside target cells

21
Q

General function of thyroid hormones

A

increase the basal metabolic rate

affect protein synthesis

help regulate long bone growth

neural maturation

increase the body’s sensitivity to catecholamines (such as adrenaline) by permissiveness

22
Q

What factors increase the tonic release of TSH (3)

A

Cold
Exercise
Pregnancy

23
Q

What do somatostatins inhibit

A

TSH

24
Q

What do glucocorticoids inhibit

A

TSH and conversion of T4 to T3

25
Q

What is the effect of Thyroid hormone binding to nuclear receptors in their target cells?

A

They change transcription and translation to alter protein synthesis

26
Q

Result of altering protein synthesis

A

raises metabolic rate and promotes thermogenesis

increase hepatic gluconeogenesis, although no effect on BG providing pancreas is releasing adequate insulin

net increase in proteolysis

net increase in lipolysis

Growth - lack of TH results in retarded growth

27
Q

Grave’s disease

A

Common disease where antibodies are produced that mimic Thyroid Stimulating Hormone and continually bind to and activate the thyroid gland.

Increased TH release from gland switches off TSH release from the anterior pituitary

therefore [TSH] plasma becomes very low

Thyroid gland may be 2-3x normal size due to hyperplasia.

Hyperactivity of cells also apparent.

28
Q

What is a thyroid adenoma

A

A rare hormone-secreting thyroid tumour

29
Q

Explanation of symptoms of hyperthyroidism

A

weight loss/ heat intolerance - due to increased M.R and heat production

muscle weakness/weight loss - due to increased protein catabolism

hyperexcitable reflexes and psychological disturbances - altered nervous system function

increased HR/contractile force, high output, cardiac failure - elevated CV function. TH is permissive to epinephrine, B receptors

30
Q

Symptoms of hyperthyroidism

A

increased HR/contractile force, high output, cardiac failure

hyperexcitable reflexes and psychological disturbances

muscle weakness/weight loss

weight loss/ heat intolerance

31
Q

What is Hashimoto’s disease

A

An autoimmune attack of the thyroid gland

Most common cause of hypothyroidism due to deficiency in dietary iodine

idiopathic

32
Q

Symptoms of hypothyroidism

A

weight gain/cold intolerance

brittle nails/thin skin

Slow speech/reflexes, fatigue

slow HR/weaker pulse

33
Q

Define the term Goitre

A

Significant enlargement of the thyroid gland

34
Q

Why does Goitre occur

A

May be caused by increased trophic action of TSH on thyroid follicular cells
(hypothyroidism)

or over-activity as a result of autoimmune disease (Graves Disease)

35
Q

True or false: Goitre can occur in both hypothyroidism and hyperthyroidism

A

True

2ND YEAR SYSTEMS: Diabetes and Endocrinology (21 decks)

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