Thyroid Physiology Flashcards

1
Q

Describe the basic anatomy of the thyroid gland

A

2 lateral lobes and an isthmus

Near third tracheal ring

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2
Q

What are the three main arteries and veins of the thyroid?

A

Sup. & Inf. Arteries and Thyroid Ima artery (Inferior)

Sup. Mid & Inf thyroid veins

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3
Q

Where are thyroid hormones stored?

A

Thyroid follicles

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4
Q

What three hormones does the thyroid make and secrete?

A

Basal metabolic rate
Thyroxine (T4)
Tri-iodothyronine (T3)

Calcium homeostasis
Calcitonin

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5
Q

Describe the basic structure of T3 and T4

A
T4 = 2 Di-iodotyrosine
T3 = 1 DIT and 1 Mono-iodotyrosine
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6
Q

How does Iodine enter the colloid space? (Three transporters)

A

Basolateral membrane creates Na+ conc. gradient:

1) ATPase: Na+ out ,K+ in
2) Na-I Symporter: I- in Na+ in

Apical membrane:
3) Pendrin: I- in

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7
Q

What are some sources of iodine in the diet?

A
Milk and Dairy
Seafood
Sea salt
Supplemented foods
Fruit and Veg grown in iodine-rich soil
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8
Q

After iodine enters the colloid space, how is T4 and T3 synthesised?

A

1) Oxidation of iodide (I-) to iodine (Io)
2) Iodination of thyroglobulin

3) Coupling: MIT + DIT = T3
DIT + DIT = T4

4) T3/T4 are bound to Thyroglobulin for storage or cleaved for secretion

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9
Q

Outline the Hypothalamus-Pituitary-Thyroid axis

A

1) Cold temperature/exercise/pregnancy - Hypothalamus releases thyrotropin releasing hormone (TRH)
2) TRH signals pituitary to release TSH
3) TSH signals thyroid to synthesise and secrete T3 and T4
4) T3 and T4 create -ve feedback to pituitary and hypothalamus.

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10
Q

What is TPO?

A

Thyroid Peroxidase: catalyses iodide oxidation and MIT and DIT coupling

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11
Q

How are T3 and T4 transported in the blood?

A

Lipophilic so need to be bound to Thyroxin Binding Globulin (70%) and Albumin (30%)

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12
Q

Describe the physiological effect of T3 and T4

A

Only free T3/T4 can enter cell and have an effect

Deiodinases interconvert T4 to T3 (only T3 can bind thyroid hormone receptor)

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13
Q

Name 4 physiological actions of T3 and T4

A

1) Metabolism
2) Maturation and Differentiation
3) Neurological functions (e.g. synapse formation)
4) Growth

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14
Q

What is Congenital hypothyroidism?

A

impaired physical (dwarfism, dystrophy bones, low BMR) and neurological development due to iodine deficiency during foetal development.

Screen: TSH heel prick test

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15
Q

What are symptoms of Hypothyroidism

Hashimoto’s Thyroiditis. Iodine Deficiency

A
Fatigue and tiredness
Sensitivity to the cold
Constipation
Dry skin and hair (alopecia)
Low mood and mental slowness
Goitre
overweight/obese
Heavy periods and fertility problems
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16
Q

What are symptoms of Hyperthyroidism

Graves Disease

A
heat intolerance, sweating, warm, moist hands
weight loss despite increased appetite
diarrhoea
palpitations / rapid pulse
tiredness and weak muscles
nervousness, irritability, mood swings
and shakiness
Thirst and polyuria
itchiness
goitre
17
Q

What lab investigations are used to test thyroid function?

A

Serum TSH - If abnormal then:
T4 - Free T4 (the fraction not bound to protein)
T3 - Free T3 (the fraction not bound to protein)

18
Q

What are the treatments for Hyperthyroidism?

A

Anti-thyroid drug therapy
Radioactive Iodine
Surgery

19
Q

What are the treatments for Hypothyroidism?

A

Levothyroxine (Synthetic T4)

Liothyronine (Synthetic T3)