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(LUSUMA) Metabolism, Endocrinology & Haematology > S7) The Thyroid Gland > Flashcards

S7) The Thyroid Gland Flashcards

1
Q

Briefly, describe the structure of the thyroid gland

A

2 lobes joined by isthmus

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2
Q

Describe the location of the thyroid gland

A
  • Lies against and around front larynx and trachea
  • Below thyroid cartilage
  • Isthmus extends from 2nd to 3rd rings of the trachea
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3
Q

Thyroid is first endocrine gland to develop.

In 5 steps, describe the embryological development of the thyroid gland

A

⇒ Appears as an epithelial proliferation in floor of pharynx at base of tongue

⇒ Descends as diverticulum through thyroglossal duct

⇒ Migrates downwards in front of hyoid bone

⇒ Remains connected to tongue by thyroglossal duct

⇒ Duct degenerates and detached thyroid continues to final position

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4
Q

Describe the microscopic structure of the thyroid gland

A
  • Follicular cells arranged in spheres called thyroid follicles
  • Follicles filled with extracellular colloid, a deposit of thyroglobulin
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5
Q

The thyroid and parathyroid are distinct glands.

Describe how their cells vary in terms of hormone production

A
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6
Q

Describe how thyroid hormones are formed

A

2 tyrosine molecules are linked together with iodine at 3/4 positions on the aromatic rings

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7
Q

What is the role of thyroglobulin in thyroid hormone formation?

A

Thryoglobulin acts as a scaffold on which thyroid hormones are formed

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8
Q

What is thyroid peroxidase and what does it do?

A

Thyroid peroxidase is a membrane bound enzyme that regulates thyroid hormone synthesis

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9
Q

Thyroid peroxidase regulates 3 separate reactions involving iodide.

Identify these

A
  • Oxidation of iodide to iodine (requires H2O2)
  • Addition of iodine to tyrosine acceptor residues on the protein thyroglobulin
  • Coupling of MIT/DIT to generate thyroid hormones within the thyroglobulin protein
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10
Q

How is dietary iodine absorbed in the thyroid gland?

A
  • Iodine is oxidised to iodide before absorption in the small intestine
  • Iodide (I-) is taken up from blood by thyroid epithelial cells via a sodium-iodide symport
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11
Q

Explain the following statement: most of T4 is converted to T3 outside of the thyroid

A
  • 90% of thyroid hormone secreted is T4
  • Most T4 is converted to T3 in the liver & kidneys
  • 80% of circulating T3 is derived from T4
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12
Q

Which thyroid hormone is more biologically active?

A

Biological activity of T3 is 4x that of T4

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13
Q

How are the thyroid hormones transported in the blood?

A

T3 & T4 are transported in blood bound to the protein thyroxine-binding globulin

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14
Q

Thyroid hormones effect virtually every cell in the body and have two interconnected responses.

What are these?

A
  • Effects on cellular differentiation and development
  • Effects on metabolic pathways
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15
Q

Regulation of thyroid hormone secretion is via negative feedback.

Outline this

A
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16
Q

Describe the structure of thyroid stimulating hormone

A

Glycoprotein hormone composed of 2 non-covalently bound subunits:

  • α subunit is also present in FSH and LH
  • β subunit provides unique biological activity
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17
Q

TSH is a trigger for thyroid hormone release.

Identify the seven processes stimulated sequentially

A

⇒ Iodide uptake

⇒ Iodide oxidation (iodine)

⇒ Thyroglobulin synthesis

⇒ Thyroglobulin iodination

⇒ Colloid pinocytosis into cell

⇒ Proteolysis of thyroglobulin

⇒ Cell metabolism & growth

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18
Q

Describe the cellular mechanisms involved in the TSH induced signalling pathway for thyroid hormone secretion

A
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19
Q

Identify the three general effects of thyroid hormones on the body

A
  • Increase in BMR and heat production
  • Stimulation of metabolic pathways
  • Sympathomimetic effects
20
Q

How do thyroid hormones increase BMR and heat production?

A
  • Increases number and size of mitochondria
  • Stimulates synthesis of enzymes in respiratory chain
21
Q

Describe two metabolic pathways stimulated by thyroid hormones

A

Catabolic pathways:

  • Lipid metabolism – lipolysis and β-oxidation
  • Carbohydrate metabolism – insulin-dependent entry of glucose into cells, gluconeogenesis and glycogenolysis
22
Q

What are the sympathomimetic effects of thyroid hormones?

A

Sympathomimetic effects – increases target cell response to catecholamines by increasing receptor number

23
Q

Identify the tissue specific effects of thyroid hormones in terms of the following:

  • Cardiovascular system
  • Nervous system
A
  • CVS – increases heart’s responsiveness to catecholamines (increased cardiac output and peripheral vasodilation)
  • Nervous system – myelination of nerves & development of neurones
24
Q

Describe the function and action of thyroid hormone receptors

A
  • Function as hormone-activated transcription factors
  • Act by modulating gene expression
25
Q

In four steps, outline the cell signalling pathway induced by thyroid hormones on intracellular receptors

A
26
Q

Indicate the normal plasma levels of thyroid hormones as well as reasons for variations unrelated to thyroid disease

A
27
Q

What is goitre?

A
  • Goitre is the enlargement of the thyroid gland due to its overstimulation
  • It may accompany either hypo- or hyperthyroidism
28
Q

What is hypothyroidism?

A

Hypothyroidism is an endocrine disorder wherein the thyroid gland is underactive and produces an insufficient amount of T3/T4

29
Q

How does a patient with hypothyroidism present?

A
  • Obesity
  • Lethargy
  • Intolerance to cold
  • Bradycardia
  • Dry skin
  • Alopecia
30
Q

What are the thyroid function test results for a patient with hypothyroidism?

A
  • Low T3
  • Low T4
  • Elevated TSH
31
Q

Identify 6 possible causes of hypothyroidism

A
  • Failure of thyroid gland
  • Congenital (TSH/TRH deficiency)
  • Inadequate dietary supply of iodine
  • Autoimmunity
  • Post surgery
  • Anti-thyroid drugs
32
Q

Severe hypothyroidism in infants is referred to as cretinism.

How does this present?

A
  • Dwarfed stature
  • Mental deficiency
  • Poor bone development
  • Slow pulse
  • Muscle weakness
33
Q

Severe hypothyroidism in adults is referred to as myxedema.

How does this present?

A
  • Thick puffy skin
  • Muscle weakness
  • Slow speech
  • Mental deterioration
  • Intolerance to cold
34
Q

What is Hashimoto’s disease?

A
  • Hashimoto’s disease is an autoimmune disease resulting in destruction of thyroid follicles, leading to hypothyroidism
  • Is is 5x more common in women and goitre may be present
35
Q

What are the thyroid function test results for a patient with Hashimoto’s?

A
  • Low T3
  • Low T4
  • Elevated TSH
36
Q

How is Hashimoto’s disease treated?

A

Oral thyroid hormone (T4 – longer half-life)

37
Q

What is hyperthyroidism?

A

Hyperthyroidism is an endocrine disorder wherein the thyroid gland is overactive and produces an excess amount of T3/T4

38
Q

How does a patient with hyperthyroidism present?

A
  • Weight loss
  • Irritability
  • Heat intolerance, sweating warm vasodilated hands
  • Tachycardia
  • Fatigue, weakness
39
Q

Identify 6 possible causes of hyperthyroidism

A
  • Autoimmune
  • Toxic multinodular goitre
  • Toxic adenoma / thyroid carcinoma
  • Excessive T4/T3 therapy
  • Drugs e.g. amiodarone
  • Ectopic thyroid tissue
40
Q

What is Graves’ disease?

A
  • Graves’ disease is an autoimmune disease resulting in hyperthyroidism caused by the production of thyroid stimulating immunoglobulin (TSI)
  • TSI continuously stimulates thyroid hormone secretion outside normal negative feedback control
41
Q

How does Graves’ disease present?

A
  • Increased BMR
  • Excessive sweating
  • Decrease in body weight
  • Muscle weakness
  • Heart palpitations
  • Exopthalmos (not always)
42
Q

What are the thyroid function test results for a patient with Graves’ disease?

A
  • Increased T3
  • Increased T4
  • Very low TSH
43
Q

How is Grave’s disease treated?

A

Antithyroid drugs are used to treat overactive thyroid

44
Q

Carbimazole is the most commonly used antithyroid drug in the UK.

Describe the mechanism of its action

A
  • Carbimazole block formation of thyroid hormone
  • Carbimazole is a pro-drug and is converted to methimazole in body
  • Prevents thyroid peroxidase from coupling and iodinating tyrosines on thyroglobulin
45
Q

Which imaging technique is used to visualise the thyroid gland and how is it performed?

A

Thyroid scintigraphy – technetium-99m (99mTc) used for isotope scanning of the thyroid with a gamma camera

(LUSUMA) Metabolism, Endocrinology & Haematology (20 decks)

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