Thyroid Gland

Question 1

What is the functional unit of the Thyroid gland?

Answer 1

The follicle.

Question 2

What is Colloid? What does it contain?

Answer 2

Colloid is the fliud inside a follicle, which contains Thyroglobulin (TG), which contains tyrosine.

Question 3

What does the Thyroid gland look like? Where is it?

Answer 3

The thyroid gland is a bilobed gland, with the glands connected by the isthmus. It is just caudal to the Larynx.

Question 4

What are the two most important thyroid hormones?

Answer 4

T3 - Triiodothyronine

T4 - Thyroxine

Question 5

What is T3? What is it made from?

Answer 5

T3 is made from 1 x MIT and 1 x DIT. It is the biologically active thyroid hormone, and is secreted less than T4.

Question 6

What is T4? What is it made from?

Answer 6

T4 is made from 2 x DIT. It is a circulating pro-hormone and is converted to T3 to be active. It is secreted in larger amounts than T3.

Question 7

How do Thyroid hormones negatively feedback? Where do they do this?

Answer 7

T3 can feedback on both TSH and TRH. T4 has to be converted to T3 to feedback.

Question 8

How are thyroid hormones synthesised?

Answer 8

TSH binds to GPCR, and increase cAMP. This phosphorylates iodide pump and brings in I. I and TG are combined by TPO. T3 and T4 made, they can then be endocytosed and T3 and 4 can be excreted.

Question 9

What is thyroperoxidase? (TPO)

Answer 9

An enzyme present on the epithelial cells of follicles that aids iodide molecules and thyroglobulin joining to form T3 and T4.

Question 10

When TSH binds to follicle epithelial cell, how does it initiate iodide pump?

Answer 10

It binds to GPCR, increases cAMP, which phosphorylates iodide pump.

Question 11

What three carrier proteins can thyroid hormones be bound to in circulation?

Answer 11

Thyroid binding globulin (TBG), Thyroid binding pre-albumin (TBPA) or Albumin.

Question 12

In the circulation, which hormone (bound or unbound) is responsible for hormonal activity?

Answer 12

Free hormone (unbound).

Question 13

How do thyroid hormones act within their target cell?

Answer 13

They bind to Thyroid hormone receptor, which is attached to thyroid hormone responsive elements. This then influences the cell’s mRNA synthesis.

Question 14

What happens when unliganded Thyroid receptor is not bound by thyroid hormone?

Answer 14

When T3 is not bound, Thyroid hormone receptor is attached to Corepressors, so it does not act.

Question 15

What happens when liganded Thyroid receptor is not bound by thyroid hormone?

Answer 15

When T3 binds to Thyroid H receptor, dimerisation of retinoic acid attracts Coactivators and so it can act.

Question 16

How does TRH cause the pituitary secretion of TSH?

Answer 16

TRH attaches to GPCR, and it increases intracellular calcium and increases phospholipase C, which increases glycosylation of TSH.

Question 17

What are the main effects of thyroid hormones?

Answer 17

Innrease basal metabolic rate, Important for normal growth, changes from glucose metabolism to fat metabolism. Positive inotropes and chronotropes.

Question 18

Where are the main sites of thyroid metabolism?

Answer 18

Liver, Brain, Muscle and Kidney.

Question 19

How is T4 metabolised? (3)

Answer 19

1) Deiodinated into T3. 2) Converted into T3. 3) Conjugated and thus excreted.

Question 20

What are C cells? What do they secrete?

Answer 20

C cells are in the thyroid gland, and they secrete Calcitonin. Calcitonin decreases blood calcium.

Question 21

If there is a lack of thyroid hormone in late fetal life or early post-natal development, what issues does this rise?

Answer 21

Cretinism (mental retardation)

Question 22

What are the clinical signs of Hypothyroidism?

Answer 22

Weight gain, Bradycardia, Myxoedema, Hyperkeratosis (Scaly), ‘Sad’ expression, Lower BMR, Infertility

Question 23

What are the 3 main causes of Hypothyroidism?

Answer 23

Immune-mediated thyroiditis, Idiopathic Thyroid atrophy, Thyroid tumour (non-secreting).

Question 24

What are the clinical signs of Hyperthyroidism?

Answer 24

Weight loss, Restless, nervous, tachycardia, Infertility

Question 25

What can be the cause of hyperthyroidism?

Answer 25

Thyroid adenoma, pituitary tumour.

Question 26

What is a Goitre? What can cause it?

Answer 26

Non-neoplastic, non-inflammatory growth of Thyroid. Can be: Iodine excess, not enough iodine.

Question 27

What is primary hypothyroidism? What happens to T4, TSH and TRH levels?

Answer 27

When there is a problem with the thyroid. T4 levels are low, TSH levels are high, TRH causes an exaggerated TSH response.

Question 28

What is secondary hypothyroidism? What happens to T4, TSH and TRH levels?

Answer 28

Secondary = Pituitary problem. T4 are low, TSH is low, and TRH causes no TSH response.