Thyroid

Question 1

Where is the thyroid located?

Answer 1

Anterior to the larynx and upper trachea. Inferior to thyroid cartilage.

Question 2

Describe the structure of the thyroid

Answer 2

Two lateral lobes joined by a central isthmus

Question 3

Which type of cells are found in the thyroid?

Answer 3

Follicular (contain colloids) and parafollicular (C-cells)

Question 4

What hormones does the thyroid produce?

Answer 4

Thyroxine (T4), tri-iodothyronine (T3) and calcitonin

Question 5

Which hormones are produced by the follicular cells and what are their functions?

Answer 5

T3 & T4 - derived from tyrosine with addition of iodine. T3 is the active form, T4 is stable therefore more T4 is released and later converted to T3

Question 6

Which hormone is released by the parafollicular cells and what is its function?

Answer 6

Calcitonin. Involved in calcium metabolism (mostly in pregnant women)

Question 7

How are T3 & T4 synthesised?

Answer 7

Transport of iodide into epithelial cells, thyroglobulin produced, exocytosis, oxidation of iodine, iodination of side chain of tyrosine residues to form MIT and DIT, coupling to form T3 & T4 ~1:10

Question 8

Where are T3 & T4 stored?

Answer 8

Extracellularly in lumen of follicles as part of thyroglobulin molecules

Question 9

How are T3 & T4 secreted?

Answer 9

Endocytosis. Proteolytic cleavage of thyroglobulin occurs to release them and they diffuse into circulation

Question 10

How are T3 & T4 transported?

Answer 10

They are hydrophobic and therefore are bound to proteins (TBG, pre-albumin and albumin)

Question 11

Where is TRH released from?

Answer 11

Dorsomedial nucleus of hypothalamus

Question 12

Describe TSH

Answer 12

Glycoprotein consisting of two non-covalently linked sub units

Question 13

What effects do T3 & T4 have on metabolic rate?

Answer 13

Increase metabolic rate - increased glucose uptake, stimulate mobilisation and oxidation of FA, stimulate protein metabolism. Mainly catabolic

Question 14

Why are T3 & T4 important for growth and development?

Answer 14

Affect bone mineralisation, synthesis of heart muscle protein, development of cellular processes of nerves, hyperplasia of cortical neurones and myelination. Absence = cretinism.

Question 15

What is the mechanism of action of T3 & T4?

Answer 15

Within the cell, bind to a receptor and cause conformational change that unmasks DNA-binding domain, increase rate of transcription, stimulates oxidative energy metabolism, increases cell activity

Question 16

How can T4 be converted into T3?

Answer 16

Removal of the 5’-iodide. Helps regulate amount of free hormone.

Question 17

What does removal of the 3’-iodide produce?

Answer 17

Inactive rT3

Question 18

What causes Hashimoto’s disease?

Answer 18

Destruction of follicles or production of an antibody that blocks the TSH receptor

Question 19

How is Hashimoto’s disease treated?

Answer 19

Oral thyroxine - over treating it could lead to hyperthyroidism

Question 20

What are the effects of hypothyroidism?

Answer 20

Cold intolerance, decreased BMR, mood swings, weight gain, lethargy, bradycardia, NMS weakness, dry/flaky skin, alopecia

Question 21

Aside from Hashimoto’s disease, what are other causes of hypothyroidism?

Answer 21

Post-surgery, radioactive iodine, anti-thyroid drugs, secondary (lack of TSH), congenital, iodine deficiency

Question 22

What are the effects of hyperthyroidism?

Answer 22

Heat intolerance, increased BMR, increased oxygen consumption, eye signs, weight loss, hyperactivity, tachycardia, skeletal and cardiac myopathy, osteoporosis

Question 23

What is Grave’s disease?

Answer 23

Autoimmune disease in which antibodies are produced that stimulate TSH receptors on follicle cells resulting in increased production/release of T3 and T4

Question 24

How would you treat people who have Grave’s disease?

Answer 24

Carbimazole - inhibits addition of iodine into thyroglobulin

Question 25

Aside from Grave’s disease, what are the other causes of hyperthyroidism?

Answer 25

Toxic multinodular goitre, excessive T3 & T4 therapy, excess iodine, thyroid carcinoma, ectopic thyroid tissue

Question 26

Outline the process through which T3 & T4 are stimulated

Answer 26

TRH (hypothalamus) –> TSH (anterior pituitary gland) –> T3/T4 (follicular cells of thyroid)

Question 27

Draw a diagram to show the negative feedback mechanisms in place for thyroid hormone

Answer 27

Question 28

Draw a table to show levels of TSH and free T4 for euthyroid, hyperthyroid and hypothyroid

Answer 28