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Endocrine > Thyroid Physiology and Anatomy > Flashcards

Thyroid Physiology and Anatomy Flashcards

1
Q

Thyroid Gland

A

Largest endocrine gland
Only endocrine gland which is superficial and allows palpation

Requires substances from the environment to synthesise hormone

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2
Q

Thyroid gland embryology

A

4th week of human embryogenesis

  • Midline thickening at back of the tongue and stretches downwards
  • Migrates in front of larynx and comes into close proximity with parathyroid glands
  • Comes into contact with cells from lower part of the pharynx (become C-cells - produce calcitonin)

End of 2nd month

  • 2 lobes joint at isthmus in front of trachea
  • Thyroglossal duct loses contact (does not occur in 15% of population0

Week 12
- Primitive follicles become visible as simple epithelium surrounding central lumen

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3
Q

Thyroid Gland point of origin remnant

A

Point of origin in tongue persists as foramen caecum

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4
Q

Failure of development

A

Congenital hypothyroidism

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5
Q

Under or over migration

A

Lingual or retrosternal thyroid

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6
Q

Failure of thyroglossal duct to recess

A

thyroglossal cyst

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7
Q

Thyroid gland vertebral levels

A

C5, C6, C7 and T1

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8
Q

Secretions of. thyroid gland tissues

A

Thyroxine (T4)
Tri- iodothyronine (T3)
Calcitonin

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9
Q

parathyroid gland secretions

A

4 parathyroid glands

Secrete parathyroid hormone (PTH)

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10
Q

Thyroid gland shape

A

‘butterfly shape’

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11
Q

Thyroid gland innervation

A

Autonomic nerve supply

Parasympathetic
- vagus nerves

Sympathetic
- superior, middle and inferior ganglia of sympathetic trunk

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12
Q

Thyroid gland blood supply

A

Superior and inferior thyroid arteries

- branch of external carotid

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13
Q

Thyroid gland venous drainage

A

Superior/ middle thyroid vein
- drains to internal jugular

inferior thyroid vein
- drains to brachiocephalic vein

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14
Q

Thyroid gland support

A

Supported by ligaments and strap muscles

Posteromedial aspect of the gland is attached by posterior suspensory ligament (Berry ligament)

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15
Q

basic function unit of thyroid gland

A

Follicle

  • colloid surrounded by follicular cells
  • parafollicular cells

Richly vascularised and innervated

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16
Q

Control of thyroid gland

A

Controlled by negative feedback loop

Thyroid is controlled by TSH from the pituitary gland. (in turn regulated by TRH from hypothalamus)

T3 predominantly completes the. negative feedback loop by suppressing the production of TSH and TRH

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17
Q

Underactive thyroid

A

High TSH as compensatory

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18
Q

Thyroid Hormone Synthesis (6)

A
  1. Thyroglobulin synthesis (thyroxine rich protein- exclusively made by follicular cells)
  2. Uptake and concentration of iodide
  3. Oxidation of iodide to iodine.
  4. Iodination of thyroglobulin
  5. Formation of MIT and DIT
  6. Secretion
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19
Q

Synthesis and storage of T3 and T4 (4)

A
  1. Iodine taken up by follicle cells
  2. Iodine attached to tyrosine residues on thyroglobulin to form MIT and DIT
  3. Coupling of
    MIT + DIT =T3
    DIT + DIT = T4
  4. Stored in colloid thyroglobulin till required
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20
Q

Effect on carbimazole and Propylthiouracil on synthesis and storage of T3 and T4

A

Step 3 can be inhibited:
Iodine unable to attach to tyrosine residues on thyroglobulin - No formation of MIT and DIT

Used to treat hyperthyroidism

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21
Q

Thyroid Hormone- T4

A

Thyroxine
90% of thyroid hormones secreted
Converted to T3 by liver and kidney
Not biologically active

22
Q

Thyroid Hormone - T3

A

Tri-iodothyronine
10% of thyroid hormones secreted
Major biologically active thyroid hormone

23
Q

Thyroid hormone transport

A

Bound to serum proteins

  • thyroxine binding globulin (TBG)
  • Thyroxine binding pre-albumin (TBPA)
  • Albumin

Only unbound hormones can enter cells

24
Q

Thyroid binding globulin (TBG)

A

T4 and T3 are almost entirely bound by plasma proteins

T3 has a more rapid onset and offset of actions as its less avidly bound

Only free/unbound hormone is available to tissues

Metabolic state correlates more freely with the total concentration in the plasma

Concentration of total hormone does not necessarily vary directly with that of free hormones .

25
Increased TBG | aetiology
Increases total T4 (but not free T4) ``` Pregnancy Newborn state OCP and other sources. of oestrogen Hepatitis A Chronic active hepatitis Biliary cirrhosis Acute intermittent porphyria Genetics Clofibrate Heroin ```
26
Decreased TBG | aetiology
Decreases total T4 (but not free T4) ``` Androgens Karge doses of glucocorticoids Cushing Syndrome Active acromegaly Severe systemic illness Chronic liver disease nephrotic syndrome Genetics Pheytoin Carbamzepine ```
27
Effects of Thyroid Hormone (all cells, especially neurons)
Increased metabolic rate | Increased glucose uptake
28
Effect of thyroid hormone on liver tissue
Increased glycogenolysis and gluconeogenesis | Decreased glycogenesis
29
Effect of thyroid hormone on adipose connective tissue.
Increased lipolysis | Decreased lipogenesis
30
Effect of thyroid hormone on lungs
Increased breathing rate | - Helps meet increased O2 demand for aerobic cellular respiration
31
Effect of thyroid hormone on heart
Increased heart rate Increased force of contraction Helps meet increased O2 demand for aerobic cellular respiration
32
thyroid hormone physiological effect
target virtually every tissue Occurs throughout life to alter. gene transcription and protein synthesis Profound effects on - metabolism - growth - developement - reproduction - behaviours
33
Thyroid Hormone Effect: Metabolic rate
Increases metabolic rate increase number and size of mitochondria Increases oxygen use and rates of ATP hydrolysis Increase synthesis of respiratory chain enzymes
34
Thyroid Hormone: Thermogenesis
Increases thermogenesis ~ 30 % of temperature regulation due to thyroid hormone thermogenesis
35
Thyroid Hormone: Carbohydrate metabolism
Increases blood glucose - stimulation of glycogenolysis and gluconeogenesis Increased insulin dependent glucose uptake into cells
36
Thyroid Hormone: Lipid metabolism
Mobilise fats from adipose tissue Increases fatty acid oxidation in tissues
37
Thyroid Hormone; Protein Metabolism
Increases protein synthesis
38
Thyroid Hormone: Growth and Development
Growth Hormone releasing hormone (GHRH) production and secretion requires thyroid hormones Glucocorticoid-induced GHRH release also dependent on thyroid hormones GH / Somatomedins require presence of thyroid hormone for activity
39
Thyroid hormone: development of foetal and neonatal brain
Myelinogenesisi and axonal growth require thyroid hormone
40
Thyroid Hormone: CNS activity
Hypothyroidism: Slow intellectual functions Hyperthyroidism: nervousness, hyperkinesis and emotional liability
41
Thyroid Hormones Permissive Sympathomimetic Action
Thyroid hormones increase responsiveness to adrenaline and sympathetic NS neurotransmitter, noradrenaline, by increasing number of receptors CV responsiveness also increased due to this affect - increase force and rate of contraction
42
use of beta-blocker in hyperthyroidism
Need to use beta-blocker (egg, propranolol) to treat symptoms in initial stages if therapy for hyperthyroidism,
43
De-iodinase Type 1
D1 | Found in liver and kidney
44
De-iodinase type 2
D2 | Found in heart, skeletal muscle, CNS, fat, thyroid and pituitary
45
De-iodinase type 3
D3 | Found in foetal tissue, placenta and brain (except pituitary)
46
Degradation of thyroid hormones
Thyroid produces T4 and T3 which enter circulation and peripheral tissues T4 - 15% go to minor degradative pathways - 45% converted by D3 into inactive reverse T3 and then rapidly excreted - 40% converted by D1 into T3 T3 - can become biologically active - converted to Inactive T2 by D3 In the brain and pituitary thyrotroph - T4 converted to T3 by D2. T3 then contributes to negative feedback on TRH and TSH
47
Hypothyroidism
Deficiency of thyroid hormones
48
Hypothyroidism aetiology
Primary (gland) failure - may be associated with goitre Secondary to TRH or TSH - no goitre lack of iodine in diet
49
Hypothyroidism symptoms
``` Reduced BMR Slow pulse rate Fatigue, lethargy. slow response times and mental sluggishness Cold-intolerance Tendency to put on weight easily ``` In adults -myxoedema babies - Cretinism (dwarfism and limited mental functioning)
50
Hyperthyroidism Aetiology
Graves Disease Toxic adenoma Toxic nodule in MNG
51
Graves disease
Autoimmune - TSH receptor antibodies act like TSH but are unchecked by T3 and T4 Exopthalmos (bulging eyes) Goitre
52
Hyperthyroidism symptoms
``` Increased BMR Very fast pulse rate Increased nervousness and excessively emotional Insomnia Sweating and heat intolerance Tendency to lose weight easily ```

Endocrine (25 decks)

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