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Metabolism > Thyroid > Flashcards

Thyroid Flashcards

0
Q

What type of gland is the thyroid?

A

-Ductless alveolar endocrine

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1
Q

Where is the thyroid located?

A

-In the neck, situated in front of the lower larynx and upper trachea and wraps around the trachea below the cricoid cartilage

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2
Q

What shape is the thyroid gland?

A

-Butterfly shape with two lateral lobes joined by a central isthmus

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3
Q

What is the usual size of the thyroid?

A

-2-3cm in diameter

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4
Q

What two cell types are found in the thyroid?

A
  • Follicular

- Parafollicular (c-cells)

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5
Q

How are follicular cells arranged?

A

-In numerous functional units called follicles separated by connective tissue

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6
Q

What is the structure of the follicles?

A

-Spherical, lined with epithelia surrounding a colloid filled centre

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7
Q

Where are the parafollicular cells located in the thyroid?

A

-In the connective tissue

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8
Q

How does the thyroid receive/drain its blood supply?

A
  • Superior/inferior thyroid arteries
  • Drainage via superior, middle and inferior veins
  • Rich lymphatic system
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9
Q

What are the three hormones produced by the thyroid?

A
  • T3->triiodothyronine
  • T4-> thyroxine
  • Calcitonin
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10
Q

Where is calcitonin produced?

A

-Parafollicular cells

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11
Q

What are T3 and T4 synthesised from?

A

-Tyrosine with the addition of iodine

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12
Q

Are T3 and T4 water-soluble or lipid-soluble?

A

-Lipid-soluble

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13
Q

Where are T3 and T4 produced?

A

-Follicular cells

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14
Q

Describe iodine uptake into the follicular cells

A
  • Follicular epithelial cells contain transporters
  • These transporters create an iodide trap at the basolateral membrane by actively pumping Na+ into the extra cellular fluid creating a Na+ gradient between the ECF anf the follicular cells
  • As Na+ moves back into the cell I- is coupled with it
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15
Q

What effect do the transporters have on the follicular epithelia cell membrane?

A

-Cause it to be polarised

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16
Q

What is iodine uptake dependant upon?

A

-Iodine from the diet

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17
Q

How and where does iodine become activated?

A
  • Activation occurs at the apical membrane

- Activated into a reactive form by a peroxidase enzyme

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18
Q

What happens to the iodine in follicular epithelial cells once it has become activated?

A

-Associates with thyroglobulin which is rich in tyrosine and Iodine is coupled to the tyrosine and stored as colloid

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19
Q

Where is thyroglobulin synthesised?

A
  • Synthesised in ribosomes

- Glycosylated in ER and packaged into secretory vessels by golgi

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20
Q

Where is thyroglobulin stored?

A

-Vesicles in follicular cells

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21
Q

What modifications occur to the thyroglobulin:iodine complex in the lumen of the follicle in order to produce T3 and T4?

A
  • Tg:I complex oxidised to produce iodinating species
  • Iodination of the side chain residues of tyrosine in thyroglobulin forms mono-iodotyrosine and di-iodotyrosine
  • Coupling of di-iodotyrosine with mono-iodotyrosine and di-iodotyrosine produced T3 and T4 respectively
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22
Q

Where are T3 and T4 stored?

A

-Extracellularly in the lumen of the follicles as part of thyroglobulin

23
Q

How are T3 and T4 secreted?

A
  • Thyroglobulin taken up into epithelial cells from lumen by endocytosis
  • Proteolytic cleavage of thyroglobulin releases T3 and T4 which diffuse from the epithelial cells into the circulation
24
Q

How are T3 and T4 transported?

A
  • Lipid-soluble and hydrophobic thus transported in the blood bound to proteins, thyronine-binding protein and albumin
  • <1% is free in solution which is the biological active form
25
Q

Is T3 or T4 in greater free proportions and why? What effect does this have on the hormone?

A
  • T3 as it has a lower affinity for thyronine-binding hormone
  • Consequently T3 has a shorter half-life
26
Q

What happens to T3 and T4 levels during pregnancy?

A
  • Relative increase as during pregnancy thyronine-binding globulin production is increased, resulting in a decrease in free T3/T4
  • Due to negative feedback TRH and TSH increase, stimulating the thyroid to produce more T3 and T4
27
Q

How is the activity of the thyroid gland controlled?

A
  • Negative feedback from the HPA
  • Hypothalamus produces thyrotropin releasing hormone, released into the HPA portal, under the influence of circulating T3/T4 levels, stress and temperature
  • TRH stimulates the anterior pituitary to secrete thyroid stimulating hormone by exocytosis from the thyrotrophs as there is a resultant increase in intracellular calcium in response to TRH
  • TSH travels in the blood to effect the follicular cells of the thyroid
  • TSH production also directly effected by circulating T3 and T4
28
Q

What effect does TSH have on thyroid follicular cells?

A

-Interactes with receptors on the surface of follicular cells and stimulates all aspects of synthesis and secretion of T3 and T4

29
Q

Besides hormone production, what other effect does TSH have on the thyroid gland? What does this effect produce?

A
  • Trophic effect that causes an increase in the size and number of follicular cells
  • Produces a goitre (enlarged thyroid gland) which may or may not be overactive
30
Q

Why is more T4 produced than T3?

A

-T3 is more potent that T4 but has a shorter half-life, thus is it beneficial to secrete more T4 and convert to T3 as needed

31
Q

Where and how does conversion of T4->T3 occur?

A

-In the peripheral tissues by a deiodination reaction

32
Q

How is T3 and T4 degraded?

A

-Complete deiodination in the liver and kidneys

33
Q

What effect do T3 and T4 have on BMR?

A

-Increase BMR by
increase number and size of mitochondria
increases O2 consumption and heat production
Increases nutrient utilisation

34
Q

What metabolic pathways do T3 and T4 stimulate?

A
  • Mostly catabolic

- Increase lipolysis, glycolysis, glycogenolysis and proteolysis

35
Q

How do T3 and T4 promotr normal growth/development of tissues?

A

-Increase synthesis of specific proteins

36
Q

What effect do T3 and T4 have on the nervous system?

A
  • Increase responsiveness of tissues to SYMPATHETIC nervous system (specifically, noradrenaline) increasing the speed of reflexes
  • Increase myelination of nerve fibres and development of neurones
  • Increase metal activity (alertness, emotional tone, memory)
37
Q

What effect do T3 and T4 have on the cardiovascular system?

A
  • Increase cardiac output

- Has direct effects on heart muscle and potentiates the effects of noradrenaline

38
Q

What effect do T3 and T4 have on skin and subcutaneous tissue?

A

-Increase turnover of proteins and glycoproteins

39
Q

What effect do T3 and T4 have on bone?

A

-Affects mineralisation

40
Q

What is the mechanism of action of T3 and T4?

A
  • Lipid-soluble so can cross pm
  • Interacts with specific high-affinity receptors located in the nucleus
  • Hormone:receptor complex undergoes conformational change exposing DNA-binding domain
  • Interaction of Hormone:receptor with DNA
  • Increases transcription of specific genes for enzymes and structural proteins
41
Q

What effects does hypothyroidism have in the newborn? Describe the effects of this disease

A
  • Cretinism
  • Severe mental retardation due to failure of CNS development
  • Diminished linear growth due to effected bone mineralisation and metabolism
  • Delayed sexual development
42
Q

Is cretinism reversible?

A

-If treated within a few weeks

43
Q

What is hypothyroidism?

A

-Undersecretion of thyroid hormones

44
Q

What are the signs and symptoms of hypothyroidism in an adult?

A
  • Cold intolerance with decreased perspiration
  • Mild weight gain
  • Bradycardia (deacreased cardiac output)
  • Constipation
  • Mood swings (anxious/depressed)
  • Slowing in cognitive ability (memory/concentration)
  • Dry skin, brittle nails, hair loss
  • Tiredness and lethargy (reduced BMR)
45
Q

What is the major cause of hypothyroidism?

A

-Hashimoto’s disease

46
Q

What causes hashiomoto’s disease?

A

-Autoimmune destruction of the thyroid follicles or production of antibodies with block TSH receptors on cells resulting in no T3 or T4 production

47
Q

How is hypothyroidism treated?

A

-Oral T4

48
Q

Besides hasimoto’s disease, what are other causes of hypothyroidism?

A
  • Post-surgery
  • Iodine deficiency
  • Rare inborn errors
  • Secondary to lack of TSH (pituitary adenoma)
49
Q

What is hyperthyroidism?

A

-Oversecretion of thyroid hormones

50
Q

What are the signs and symptoms of hyperthyroidism in the adult?

A
  • Heat intolerance (increased perspiration, moist hands)
  • Weight loss (increased lipolysis and proteolysis)
  • Tachycardia (increased cardiac output, often irregular)
  • Increased bowel movements and appetite
  • Nervousness, irritability and emotionally labile
  • Hyper-reflexivity (possible tremor of outstretched hand)
  • Exophtamalos (buldging of the eyes)
51
Q

What is the major cause of hyperthyroidism?

A

-Grave’s disease

52
Q

What is the cause of grave’s disease?

A

-Autoimmune disease by production of antibody that stimulates TSH receptor on follicular cells increasing T3/T4 production

53
Q

What are the treatments of hyperthroidism?

A
  • Carbizamole -> inhibits incorporation of iodine into thyroglobulin
  • Radioactive iodine
  • Surgery
54
Q

Besides Grave’s disease, what are other causes of hyperthyroidism?

A
  • Ectopic thyroid tissue
  • Thyroid carcinoma
  • Excess iodine
  • Other autoimmune diseases eg toxic multinodular goitre and solitary toxic adenoma

Metabolism (14 decks)

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