19. The Thyroid Gland

Question 1

What is the main function of the thyroid?

Answer 1

Formation and storage of thyroid hormone.

Question 2

What percentage of the thyroid is colloid?

Answer 2

30%

Question 3

What does colloid contain?

Answer 3

Thyroglobulin.

Question 4

What do C cells of the thyroid secrete?

Answer 4

Calcitonin.

Question 5

What is the normal blood supply to the thyroid compared to enlarged thyroid glands?

Answer 5

30-60ml/min normally, 100ml/min in enlarged thyroids.

Question 6

What is the effect of sympathetic stimulation on the thyroid gland?

Answer 6

Increased secretion of thyroid hormone.

Question 7

Explain the hypothalmic-pituitary-thyroid axis.

Answer 7

The hypothalamus secretes TRH (thyrotropin releasing hormone) which binds to Gq/11 receptors on the pituitary gland, this signals an increased release of TSH (thyrotropin stimulating hormone). TSH binds to TSH receptor on the basolateral membrane of thyroid follicular cell. This leads to stimulation of iodine uptake, production and release of idothyronines and promotion of thyroid growth

Question 8

What is the short loop of inhibition of the thyroid gland?

Answer 8

High T3/4 causes negative feedback on the pituitary gland so it secretes less TSH and less T3/4 is produced.

Question 9

What is the long loop of inhibition in the thyroid gland?

Answer 9

High T3/4 levels inhibit the hypothalamus so it releases less TRH, this reduces secretion of TSH and therefore lowers T3/4 production.

Question 10

What would high TSH levels suggest?

Answer 10

An under active thyroid.

Question 11

How does uptake of iodine in the thyroid work?

Answer 11

Follicular cells concentrate iodine using an iodine trap. The iodine is dependent on the diet.

Question 12

How is iodine activated in the thyroid gland?

Answer 12

By peroxidase enzyme.

Question 13

What are the full names of the hormones produced by the thyroid?

Answer 13

Tetra-iodothyronine/ thyroxine (T4) and tri-iodithyronine.

Question 14

Why are different amounts of T3 and T4 made?

Answer 14

More T4 is made as it has a longer half life but T3 is more potent. T4 is converted to T3 by deiodination for action.

Question 15

What percentage of thyroid hormones are bound?

Answer 15

> 99%.

Question 16

What do thyroid hormones bind to?

Answer 16

Mostly to thyronine binding globulin (TBG) but a little to albumin and pre-albumin.

Question 17

How are T3/4 degraded?

Answer 17

By the removal of iodine by the liver and kidney.

Question 18

Why is the half life of T4 greater than that of T3?

Answer 18

Because it has a greater affinity for binding proteins than T3.

Question 19

What are the general effects of thyroid hormones?

Answer 19

Increase basal metabolic rate by increasing the size and number of mitochondria, increasing oxygen consumption and heat production and increasing nutrient utilisation. It also stimulates most metabolic pathways, promotes normal growth and development of tissues, increases responsiveness of tissues to sympathetic nervous system and various hormones.

Question 20

How do thyroid hormones affect the nervous system?

Answer 20

Increased myelination of nerve fibres, increased speed of reflexes and increased mental activity.

Question 21

How do thyroid hormones affect the cardiovascular system?

Answer 21

They increase cardiac output.

Question 22

How do thyroid hormones affect the skin and subcutaneous tissue?

Answer 22

Increased turnover of proteins and glycoproteins. Without thyroid hormones, the skin becomes saggy.

Question 23

What does hypothyroidism in new borns cause? Give details.

Answer 23

Cretinism. Severe mental retardation due to failure in CNS development, coarse features and a protruding tongue, diminished linear growth and delayed sexual development.

Question 24

What are the signs and symptoms of hyperthyroidism?

Answer 24

Heat intolerance, increased perspiration,weight loss, tachycardia, increased bowel movements - including diarrhoea, nervousness, irritability, emotional lability, hyper reflective - tremor of hands and eye signs of exophthalmos (eyes stick out).

Question 25

What is the major cause of hyperthyroidism?

Answer 25

Grave’s disease, an autoimmune disease that stimulates the TSH receptor on follicle cells.

Question 26

How is hyperthyroidism treated?

Answer 26

Carbimazole to inhibit incorporation of iodine into thyroglobulin, radioactive iodine or surgery to remove the thyroid.

Question 27

Why must the surgeon be careful when performing a thyroidectomy?

Answer 27

The parathyroid glands are on the outer capsule of the thyroid so they mustn’t remove them otherwise the patient won’t be able to control Ca levels.

Question 28

What causes exophthalmos?

Answer 28

Accumulation of adipose tissue behind the eye in Graves’ disease.

Question 29

What are the signs and symptoms of hypothyroidism?

Answer 29

Cold intolerance, decreased perspiration, mild weight gain, bradycardia, constipation, mood swings - anxiety and depression, poor concentration, poor memory, oedema, dry skin, brittle nails and some hair loss.

Question 30

What is the major cause of hypothyroidism?

Answer 30

Hashimoto’s disease, autoimmune disease that cause destruction of thyroid follicles and produces an antibody that blocks the TSH receptor in follicle cells.

Question 31

How is hypothyroidism treated?

Answer 31

With oral T4 (100-200ug/day).

Question 32

How do we know if the levels of T4 being used to treat hypothyroidism are correct?

Answer 32

The secretion of TSH will return to normal.

Question 33

What is the normal weight of the thyroid compared to abnormal growth weight?

Answer 33

Normally around 20g but abnormal goitres can weigh 100g.