Thyroid Physiology

Question 1

What is the arterial supply to the thyroid?

Answer 1

Superior thyroid artery
-From external carotid

Inferior thyroid artery
-From thyrocervical trunk

Occasional supply from IMA (10% of population)

Question 2

What is the venous drainage of the thyroid?

Answer 2

Superior, middle and inferior thyroid veins

Question 3

What 3 hormones are produced by the thyroid gland and what are they involved with?

Answer 3

Thyroxine (T4) and
Tri-iodothyronine (T3) (basal metabolic rate)
Calcitonin (calcium homeostasis)

Question 4

In what part of the thyroid gland are the thyroid hormones made and stored?

Answer 4

Follicles

Question 5

What cells of the thyroid gland produce calcitonin?

Answer 5

Parafollicular C cells

Question 6

What are the 6 steps of thyroid hormone synthesis?

Answer 6

Occurs in follicular cells

1) Thyroglobulin synthesised- tyrosines on surface
2) Uptake and concentration of iodide
3) Oxidation of iodide into iodine catalysed by TPO
4) Iodine is conjugated to 1 or 2 tyrosines on the thyroglobulin molecules. Forms either Mono or Di-iodotyrosine (MIT or DIT)
5) MIT and DIT join together to form either T3 or T4.
6) When needed, T3 and T4 taken up into the follicle cell into vesicle attached to lysosome which secretes protease. Protease separate the thyroglobulin ( via hydrolysis) which re-enters the colloid via pendrin to be recycled. T3 and T4 are secreted into the blood.

Question 7

Describe the regulatory process of T3 and T4 secretion

Answer 7

Hypothalamus releases TRH which acts on anterior pituitary which then releases TSH which stimualates thryoid to produce T3 and 4.
Via a negative feedback loop, T3 and 4 act on hypothalamus and anterior pituitary to decrease stimulation of T3 and T4 secretion.

Question 8

How does TSH increase production of T3 and T4?

Answer 8

Increases action at every stage of T3 and T4 production in the follicles.

Question 9

How are thyroid hormones transported in the blood?

Answer 9

Lipophilic so can’t dissolve in blood.
70% bound to thyroxin binding globulin (TBG)
30% bound to albumin
(TBG has a higher affinity for T4)

Question 10

Which is the active thyroid hormone? Which is produced in higher supply?

Answer 10

T3 is active

T4 is inactive and produced in large quantities, must be converted to T3 to be used.

Question 11

How do T3 and T4 enter the cell?

Answer 11

Only free T3 and 4 can enter the cell. Binding proteins release the hormones when required.

Question 12

Which receptor receives thyroid hormone?

Answer 12

T3r- receives T3

Question 13

What converts T4 to T3?

Answer 13

Deiodinases (enzymes)

Can activate or inactivate thyroid hormone.

Question 14

What are the different types of deiodinases and their roles?

Answer 14

Type 1: activate T4 by converting it to T3 by inner and outer ring deiodination. (exist on cell surface of most cells)

Type 2: Activates T4 by turning it to T3 (intracellular, CNS, brown fat, placenta, skeletal and cardiac muscle)

Type 3: Inactivating, turns T4 into reverse T3 (rT3) or T3 into T2. (placenta and CNS)

Question 15

How do T3 and T4 enter the and act on the cell?

Answer 15

Via diffusion or by MCT8(10) transporter.

T3 (T4 converted first by deiodinases) binds to receptor and is then taken to the nucleus where it binds to DNA and alters protein synthesis.

Question 16

What are the physiological actions of thyroid hormone?

Answer 16

Metabolism- affects basal metabolic rate
Maturation and differentiation- bone, lung and brain tissue
Neurological function- synapse formation, myelinogenesis, neuronal outgrowth
Growth- Regulated by GH

Question 17

What are the metabolic actions of thyroid hormone?

Answer 17

Increases basal metabolic rate by increasing O2 consumption.
Increases O2 consumption by increasing ventilation, heart rate, myocardial activity (therefore CO) and increased metabolism of substrates.

Question 18

What are the effects of increased metabolism?

Answer 18

Increased CO2 therefore increased increased ventilation, increased urea, renal function. Decreased muscle and adipose tissue.

Question 19

What thyroid function test results would indicate hyperthyroidism?

Answer 19

High T4, low TSH (low TSH should normally cause low T4)

Question 20

What thyroid function test result would indicate hypothyroidism?

Answer 20

Low T4, High TSH (normally high TSH should cause high T4)

Question 21

What would low T4 and low TSH (or high T4 and high TSH) indicate and why?

Answer 21

Anterior pituitary gland problem (low T4 should normally stimulate secretion of TSH and high T4 should cause low TSH)

Question 22

How does hypothyroidism from iodine deficiency produce a goitre?

Answer 22

Reduced T4 stimulates anterior pituitary to produce TSH which stimulates thyroid gland to produce T4, however it is unable due to iodine deficiency. This results in thyroid gland hypertrophy and a goitre.

Question 23

How does hyperthyroidism produce a goitre?

Answer 23

(Grave’s disease only) Thyroid Stimulating Immunoglobulin (TSI) produced which mimics TSH so stimulates thyroid gland to produce T4. Increased T4 causes anterior pituitary to release less TSH however TSI is still continually stimulating thyroid gland to produce T4. Over stimulation results in hypertrophy and goitre.

Question 24

What are the signs and symptoms of hyperthyroidism?

Answer 24

Heat intolerance, sweating, weight loss despite increased appetite, diarrhoea, palpitations, fatigue and weak muscles, nervousness, irritability, thirst, polyuria, goitre (Grave’s disease only) protruding eyes (Grave’s only)
Symptoms due to increased metabolic rate.

Question 25

What are the signs and symptoms of hypothyroidism?

Answer 25

Fatigue, cold sensitivity, constipation, dry skin and hair (alopecia), low mood, mental slowness, goitre, weight gain, heavy periods and fertility problems.
Symptoms due to low metabolic rate.

Question 26

What are the causes of hyperthyroidism?

Answer 26

Grave’s disease

Question 27

What are the causes of hypothyroidism?

Answer 27

Iodine deficiency, hasimoto’s thyroiditis (autoimmune)

Question 28

What is used to treat hypothyroidism?

Answer 28

Levothyroxine, synthetic T4. Metabolised to T3 in body as required.