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MD2S3: Physiology 203 > Thyroid Hormones > Flashcards

Thyroid Hormones Flashcards

1
Q

Which hormones are secreted by the Thyroid Gland?

A

T4
T3
Calcitonin

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2
Q

What is T4?

A

Prohormone

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3
Q

Why is T4 converted to T3?

A

To exert their action
T3 is more biologically active than T4

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4
Q

Compare T4 and T3

A

T3 is more biological active
T3 has greater affinity for thyroid hormone receptors
T3 has a shorter half-life

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5
Q

What is calcitonin?

A

Hormone responsible for calcium homeostasis

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6
Q

Where is calcitonin produced?

A

Parafollicular C cells

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7
Q

Where is reverse T3 (rT3) produced?

A

Made outside thyroid gland by removal of iodine atom from T4

Constitutes 1% of circulating throid hormone

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8
Q

What are the steps in thyroid hormone synthesis?

A

Step 1: Iodide uptake
Step 2: Oxidation
Step 3: Organification
Step 4: Generation of T3 and T4
Step 5: Release of T3 and T4 from thyroglobulin into circulation

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9
Q

Describe the stages of Step 2 (oxidation) of thyroid hormone synthesis

A

Thyroid Peroxidase (TPO) oxidises iodide to active form iodine

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10
Q

Describe the stages of Step 3 (organification) of thyroid hormone synthesis

A
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11
Q

Describe the stages of Step 4 (Generation of T3 and T4) of thyroid hormone synthesis

A

TPO combines MIT and DIT to form T3 or combine two DITs to form T4

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12
Q

Describe the stages of Step 5 (Release of T3 and T4 from thyroglobulin into circulation) of thyroid hormone synthesis

A

Tg is endocytosed from the lumen into the follicular cell
Tg is recycled for additional iodination

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13
Q

How does deiodination occur?

A

T4 is deiodinized by 5’-deiodinase in peripheral tissues

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14
Q

What are the products of deiodination?

A

T3
rT3
Inactive degradation products

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15
Q

What is rT3?

A

Modulates action of active T3 by competing for receptor binding

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16
Q

What is the role of rT3?

A

Prevents the body from reacting strongly to T3

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17
Q

What is the Wolff-Chaikoff effect?

A

Excess iodide can inhibit thyroid peroxidase and reduce the thyroid’s ability to produceT3 and T4

Iodide is the form in which iodine is transported

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18
Q

How is the Wolff-Chaikoff effect limited?

A

Excess iodide is excreted from the urine

19
Q

How are thyroid hormones transported?

A

99% of T3 and T4 bind to thyroid-binding globulin (TBG) proteins
Remainder is free in blood stream and biologically active

20
Q

What may cause an increase in TBG levels?

A

Pregnancy
Oral contraceptive use

Estrogen stimulate TBG production in liver

21
Q

What are the effects of TBG levels increasing?

A

Bound and total thyroid hormones rise
Free T3 and T4 are constant

22
Q

What may cause a decrease in TBG levels?

A

Hepatic failure (in patients using corticosteroid drugs)

23
Q

What are the effects of TBG levels decreasing?

A

Bound and total thyroid hormones fall
Free T3 and T4 are constant

24
Q

How do T3 and T4 cross the cell membrane?

A

Transmembrane thyroid hormone transporters

T3 and T3 are lipophilic yet cannot cross the plasma membrane

25
Q

Where do thyroid hormones exert their effect?

A

Bind to nuclear receptors to modulate gene expression

26
Q

How do thyroid hormones exert metabolic effects?

A

Upregulate Na+/K+-ATPase
Promotes cellular uptake of glucose
Increases formation and catabolism of proteins

27
Q

What effect does upregulating Na+/K+-ATPase have?

A

Increases cellular oxygen consumption
Increase heat production
Increases basal metabolic rate

28
Q

What effects do thyroid hormones have on the heart?

A

Rapid pulse
Increased Cardiac Output

29
Q

Why can thyroid affect the heart?

A

Upregulate β1-adrenergic receptors in the heart which increases response to sympathetic stimulation

30
Q

What role does thyroid have in fetal development?

A

Enhances brain and bone development

31
Q

How does the thyroid hormone enhance bone development?

A

Enhances formation and growth of osteoblasts
Acts on growth plate chondrocytes

32
Q

How does the thyroid hormone enhance brain development?

A

Stimulates synapse and myelin fromation
Stimulates growth of dendrites and axons on neurons
Regulates neural crest cell migration and differentiation

33
Q

Describe the pathway of thyroid hormone secretion

A
34
Q

What is Graves Disease?

A

Autoimmune condition resulting in hyperthyroidism

35
Q

What causes Graves Disease?

A

Abnormal thyroid-stimulating antibodies (TSI) bind to TSH receptors on thyroid follicular cells
Causes persistent thyroid hormone synthesis

36
Q

Symptoms of Graves Disease

A

High excitability
Heat Intolerance
Increased sweating
Weight loss
Muscle weakness
Diarrhea
Nervousness or other psychic disorders
Fatigue with inability to sleep
Tremor of the hands
Exophthalmos

37
Q

What is myxedema?

A

Total lack of thyroid function in adults

38
Q

Symptoms of myxedema

A

Swelling of face (due to hyaluronic acid and chondroitin sulfate causing interstitial fluid accumulation)
Husky and Slow voice
Blood cholesterol increases and atherosclerosis develops

39
Q

How does iodine deficiency cause goiter development?

A

Thyroglobulin formed but no thyroid hormones
TSH secretion remains uninhibited enabling hyperplasia and growth of thyroid gland

40
Q

What is Thyroiditis/Hashimoto disease?

A

Antibody develops against thyroglobin which destroys thyroid cells
Causes hypothyroidism

41
Q

What is Cretinism?

A

Stunted physical and mental growth due to untreated congenital deficiency of thyroid hormones

42
Q

What causes cretinism?

A

Lack of thyroid hormone or iodine during prenatal, neonatal or childhood stages

43
Q

Effects of cretinism

A

Sluggish movement
Retarded physical and mental growth
Obese, stocky, short appearance
Tongue becomes large

MD2S3: Physiology 203 (13 decks)

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