Thyroid Physiology

Question 1

Describe the basic anatomy of the thyroid gland

Answer 1

2 lateral lobes and an isthmus

Near third tracheal ring

Question 2

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

Answer 2

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

Sup. Mid & Inf thyroid veins

Question 3

Where are thyroid hormones stored?

Answer 3

Thyroid follicles

Question 4

What three hormones does the thyroid make and secrete?

Answer 4

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

Calcium homeostasis
Calcitonin

Question 5

Describe the basic structure of T3 and T4

Answer 5

T4 = 2 Di-iodotyrosine
T3 = 1 DIT and 1 Mono-iodotyrosine

Question 6

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

Answer 6

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

Question 7

What are some sources of iodine in the diet?

Answer 7

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

Question 8

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

Answer 8

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

Question 9

Outline the Hypothalamus-Pituitary-Thyroid axis

Answer 9

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.

Question 10

What is TPO?

Answer 10

Thyroid Peroxidase: catalyses iodide oxidation and MIT and DIT coupling

Question 11

How are T3 and T4 transported in the blood?

Answer 11

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

Question 12

Describe the physiological effect of T3 and T4

Answer 12

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

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

Question 13

Name 4 physiological actions of T3 and T4

Answer 13

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

Question 14

What is Congenital hypothyroidism?

Answer 14

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

Screen: TSH heel prick test

Question 15

What are symptoms of Hypothyroidism

Hashimoto’s Thyroiditis. Iodine Deficiency

Answer 15

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

Question 16

What are symptoms of Hyperthyroidism

Graves Disease

Answer 16

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

Question 17

What lab investigations are used to test thyroid function?

Answer 17

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

Question 18

What are the treatments for Hyperthyroidism?

Answer 18

Anti-thyroid drug therapy
Radioactive Iodine
Surgery

Question 19

What are the treatments for Hypothyroidism?

Answer 19

Levothyroxine (Synthetic T4)

Liothyronine (Synthetic T3)