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2.4 BRS Endocrinology > Thyroid Gland > Flashcards

Thyroid Gland Flashcards

1
Q

What is the main anatomy of the thyroid gland?

A
  1. Thyroid cartilage
  2. Pyramid
  3. Thyroid gland —> 2 lobes
  4. Isthmus —> connect 2 lobes
  5. Parathyroid glands —> superior and anterior
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2
Q

What is the histology of the thyroid gland?

A
  1. Follicles —> colloid surrounded by follicular cells
  2. Parafollicular cells
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3
Q

Which nerve runs close to the thyroid gland?

A

Left recurrent laryngeal nerve —> vocal cords

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4
Q

What is the embryology of the thyroid gland?

A
  • Thyroglossal duct develops from base of tongue
  • Divides into 2 lobes
  • Duct disappears leaving foramen caecum
  • Thyroid gland being developing at 7 weeks
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5
Q

What are the 5 congenital anomalies of thyroid gland development?

A
  1. Thyroid agenesis —> no thyroid
  2. Lingular thyroid —> too high
  3. Retrosternal goitre —> too low
  4. Thyroglossal cyst —> lump
  5. Thyroglossal fistula —> hole
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6
Q

How does the thyroid gland produce T3 and T4? (6)

A

Blood —> Follicular cells:
- TSH bind to TSH-R on membrane —> stimulate TG
and TPO release
- Na+ and I- in via symport

Follicular cells:
- TG (thryoglobulin) released from nucleus
- TPO (thyroid peroxidase) released

Follicular cell —> Colloid:
- TG and TPO in
- I- in via antiport with Cl-

Colloid:
- I- to I via oxidation —> iodination of TG
- TPO + H2O2 —> catalyse iodination and coupling

  • TG + I —> MIT (3-monoiodotyrosine)
    TG + 2 I —> DIT (3,5-diiodotyrosine)
  • MIT + DIT —> T3
    DIT + DIT —> T4

Colloid —> Follicular cells:
- T3/T4 in via endocytosis
- T3/T4 vesicles bind to lysosomes

Follicular cells —> Blood:
- T3/T4 released via exocytosis

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7
Q

Which 2 hormones does the thyroid gland produce?

A
  1. T3 (triiodothyronine)
  2. T4 (thyroxine)
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8
Q

What is T3?

A

Triiododthyronine (3 Is)
- MIT + DIT
- Bioactive when 2 Is on MIT
Reverse T3 (inactive) when 2 Is on DIT —> produced
by T4 deiodination in different place
- 20% direct thyroidal secretion
80% deiondinated T4

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9
Q

What is T4?

A

Thyroxine (4 Is)
- DIT + DIT
- Prohormone —> converted to T3
- Main product of thyroid gland

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10
Q

What happens to most of T4, how and why?

A

Converted to T3 (active)
- Deiodination via deiodinase enzyme
- T4 longer half-life (7-9 vs 2 days) because inactive
- Produced active or reverse T3
- 80% of T3

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11
Q

How are T3 and T4 transported round the body?

A

Bind to plasma proteins in blood
- TBG (thyroid-binding globulin) —> 70-80%
- Albumin —> 10-15%
- Transthyretin (prealbumin)

Some unbound —> bioactive
- 0.05% T4
- 0.5% T3

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12
Q

How to T3 and T4 affect cells?

A

T3 binds to TRs (thyroid hormone receptors) on TREs (thyroid response elements) in nucleus —> alter gene expression —> new proteins synthesised

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13
Q

What is the importance of thyroid hormones in a fetus?

A

Growth and development
- especially of CNS

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14
Q

What is cretinism and how is it screened?

A

Untreated congenital hypothyroidism
- Heel-prick test

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15
Q

What are the 5 functions of thyroid hormones?

A
  1. Growth
  2. CNS maturation
  3. Thermogenesis
    • inc Na+/K+ ATPase —> inc O2 consumption —>
      inc BMR —> inc heat release
  4. Metabolism
    • inc glucose absorption, glycogenolysis,
      gluconeogenesis
    • inc lipolysis (catecholamines)
    • inc protein synthesis and degradation
  5. Cardiovascular function (catecholamines)
    • inc cardiac output, heart rate, stroke volume
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16
Q

How are thyroid hormones regulated?

A
  • TRH —> inc TSH —> inc T3 and T4
  • Somatostatin —> dec TSH —> dec T3 and T4
  • T3 and T4 —> dec TSH (negative feedback)
  • I- —> dec T3 and T4 (Wolff-Chaikoff effect)
17
Q

What is the Wolff-Chaikoff effect?

A

Regulation of T3 and T4 release from the thyroid glands via rejecting large quantities of I-

18
Q

Which gender is more affected by thyroid diseases?

A

Women
- 4:1

19
Q

What is primary hypothyroidism?

A

Underactive thyroid
- Autoimmune damage - Hashimoto’s thyroiditis
- T4 dec —> all released converted to T3 (no excess)
TSH inc —> not inhibited by T3/T4 release

20
Q

What is Hashimoto’s thyroiditis?

A

Autoimmune disease —> hypothyroidism
- Antibodies bind to thyroid gland —> gradually
destroyed

21
Q

What are the 7 main symptoms of hypothyroidism?

A
  1. Deeper voice - left recurrent laryngeal nerve
  2. Depression/tiredness - CNS
  3. Cold intolerance - thermogenesis
  4. Appetite dec but weight inc - metabolism
  5. Constipation - metabolism
  6. Bradycardia - cardiovascular
  7. Myxoedema coma - if untreated —> severe
22
Q

What medical emergency can hypothyroidism lead to?

A

Myxoedema coma

23
Q

How can hypothyroidism be treated pharmacologically? (3)

A
  1. Levothyroxine (synthesised T4)
    • oral tablet —> 100 µg once daily (dose adjusted to
      TSH levels)
    • side effects —> weight loss, headaches
      —> heart attack, rapid heart rate
  2. Liothyronine (synthesised T3)
    • oral tablet —> multiple times a day (T3 shorter
      half life)
  3. Combined thyroid hormone replacement (T3 + T4)
    • oral tablet
    • seen to improve wellbeing
    • side effects (toxicity) —-> palpitations, tremour,
      anxiety
      —-> TSH dec
24
Q

What is hyperthyroidism?

A

Overactive thyroid
- Autoimmune goitre - Grave’s disease —> all
- Toxic multinodular goitre
- Solitary toxic nodule —> 1

25
Q

What is Grave’s disease and how does it present? (3)

A

Autoimmune disease —> hyperthyroidism
- Antibodies bind to:
1. TSH-Rs —> smooth goitre
2. Muscles behind eye —> ecophthalmos
3. Shin soft tissue —> pretibial myxoedema

26
Q

What are the 9 symptoms of hyperthyroidism?

A
  1. Myopathy
  2. Mood swings - CNS
  3. Heat intolerance - thermogenesis
  4. Appetite inc but weight dec - metabolism
  5. Diarrhoea - metabolism
  6. Palpitations - cardiovascular
  7. Hand tremors
  8. Sore eyes
  9. Goitre
27
Q

What is goitre?

A

Swollen thyroid —> lump/swelling at the front of neck

2.4 BRS Endocrinology (9 decks)

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