10.1 - The Thyroid Gland.

Question 1

What joins the two lobes of the thyroid?

Answer 1

A central isthmus

Question 2

What’s the embryological development of the thyroid?

Answer 2

The first endocrine gland to develop

Question 3

What’s the histology of the thyroid tissue?

Answer 3

• Follicular cells arranged in spheres called thyroid follicles
• Follicles filled with colloid, a deposit of thyroglobulin
• Colloid is extracellular even though it is inside the follicle

Question 4

What do thyroid follicular and parafollicular cells produce?

Answer 4

Follicular = thyroid hormone

Parafollicular = calcitonin

Question 5

What makes up T3 and T4?

Answer 5

T3 = MIT and DIT
T4 = DIT and DIT

Question 6

What is the role of thyroid peroxidase?

Answer 6

Membrane bound enzyme that regulates 3 separate reactions involving iodine

1) oxidise iodide to iodine
2) addition of iodine to tyrosine on thyroglobulin
3) coupling of MIT and DIT to make thyroid hormones

Question 7

How is dietary iodine absorbed?

Answer 7

Iodine reduced to iodide before absorption principally in the small intestine

Iodide is taken up from the blood by thyroid epithelial cells, which have a sodium iodide symporter or iodine trap.

Question 8

How is T3 synthesised from T4?

Answer 8

Most t4 is converted from T3 in the liver and kidneys

Question 9

How does the negative feedback for thyroid hormone secretion work?

Answer 9

• hypothalamus releases TRH which goes to ant pit
• ant pit releases TSH which goes to thyroid gland
• more Thyroid hormone is released
• higher levels of thyroid hormone in blood = hypothalamus releases less TRH

Question 10

What is the structure of TSH?

Answer 10

Glycoproteins hormone with two covalently bound subunits, alpha and beta.

B subunit provides unique biological activity.

Question 11

What does TSH stimulate?

Answer 11

• iodide uptake and oxidation
• thyroglobulin synthesis
• thyroglobulin iodination
• colloid pinocytosis is into cell
• proteolysis of thyroglobulin
• cell metabolism and growth

Question 12

What are the general actions of thyroid hormone?

Answer 12

1) increase in basal metabolic rate and heat production
- increase number and size of mitochondria
- stimulate synthesis of enzymes in respiratory chain

2) stimulation of metabolic pathways
Catabolic more than anabolic
- lipid metabolism: stimulate lipolysis
- carbohydrate metabolism: stimulate insulin dependent entry of glucose into cells

3) sympathomimetic effects
- increase target cell response to catecholamines by increasing receptor number

Question 13

What does thyroid hormone do to the cardiovascular and nervous system?

Answer 13

Cardiovascular system
increased hearts responsiveness to catecholamines
- increased cardiac output (HR and contraction)
- peripheral vasodilation to carry extra heat to body surface

Nervous system
Essential for development and adult function
- myelination of nerves and neurone development

Question 14

How does thyroid hormone bind to intracellular receptors?

Answer 14

1) its lipid soluble so can enter through thyroid hormone transporters into the cell
2) goes through nuclear membrane, and binds to thyroid hormone receptor on DNA. Binding relies repression of gene transcription and the gene is now expressed.
3) thyroid hormone receptor is pre bound to specific DNA sequences on DNA called hormone response elements (HRE) in promoter region of thyroid regulated genes

Question 15

What are some examples of thyroid hormone activated genes?

Answer 15

• PEPCK
• Ca2+ ATPase
• Na, K ATPase

Question 16

What’s a goitre??

Answer 16

An enlargement of the thyroid
Comes with either hypo or hyperthyroidism

Developers when thyroid gland is overstimulated

Question 17

What are the causes of hypothyroidism?

Answer 17

• Failure of thyroid gland
• TSH or TRH deficiency
• inadequate dietary supply of iodine
• radioactive iodine
• autoimmunity
• post surgery
• congenital

Question 18

What are the symptoms of hypothyroidism?

Answer 18

• obesity
• lethargy
• intolerance to cold
• bradycardia
• dry skin
• alopecia
• hoarse voice
• constipation
• slow reflexes

Infants - cretinism. Dwarf state and poor bone development. Metal deficiency.

Adults - myxedema. Thick puffy skin, intolerance to cold, mental deterioration, slow speech, muscle weakness.

Question 19

What’s hashimotos disease and whats the treatment?

Answer 19

Autoimmune disease resulting in the destruction of thyroid follicles. Leads to hypothyroidism.

Plasma

• low T3
• low T4
• elevated TSH

Treatment
- oral thyroid hormone T4

Question 20

What are the causes of hyperthyroidism?

Answer 20

• autoimmune Graves’ disease
• toxic multi modular goitre
• solitary toxic adenoma
• excess T4 or T3 therapy
• drugs (amiodarone)
• thyroid carcinoma
• ectopic thyroid tissue

Question 21

What are the general symptoms of hyperthyroidism?

Answer 21

• Weight loss
• Irritability
• Heat intolerance
• tachycardia
• fatigue
• increased bowel movement
• tumour of hands
• breathless
• bulging eyes and goitre sometimes

Question 22

What’s Graves’ disease?

Answer 22

Autoimmune disease resulting in hyperthyroidism

Caused by productions of thyroid stimulating immunoglobulin (TSI)

TSI continuously stimulates thyroid hormone secretion outside normal negative feedback control

Question 23

What’s a thyroid scintigraphy?

Answer 23

Scan of the thyroid using radioactive iodine

Question 24

What are antithyroid drugs and what are the used for?

Answer 24

Used to treat an over active thyroid e.g in Graves’ disease

Block formation of thyroid hormone
Carbimazole, a pro drug, is most commonly used in UK. Is converted to methimazole in the body.

It prevents thyroid peroxidase from coupling and iodinating tyrosine on thyroglobulin