The Thyroid Gland

Question 1

What are the two thyroid hormones?

Answer 1

T3: triiodothyronine
T4: thyroxine

Question 2

What are the two cell types which make up the thyroid gland? What are their function?

Answer 2

C (clear) cells - secrete calcitonin

Follicular cells - support thyroid hormone synthesis and surround hollow follicles

Question 3

What are thyroid follicles?

Answer 3

Spherical structures whose walls are made of follicular cells. The centre of the follicle is filled with colloid - a sticky glycoprotein matrix

Question 4

What do follicular cells manufacture? what happens to these products once created?

Answer 4

The enzymes that make thyroid hormones as well as thyroglobulin. a large protein rich in tyrosine residues

The enzymes and thyroglobulin are packaged into vesicles and exported from the follicular cells into the colloid

Question 5

What do follicular cells concentrate and transport into the colloid? What does this become?

Answer 5

It actively concentrates iodine from the plasma and transports it into the colloid where it combines with tyrosine residues to from the thyroid hormones

Question 6

How does iodine enter the follicular cells from the plasma? Where does the iodine go from here and how?

Answer 6

Via a Na+/I_ transport system. Th coupling Na+ allows the cells to take up iodine against a concentration gradient

The iodine is then transported into the colloid via the pendrin transporter

Question 7

What govern the release of TH from the colloid into the plasma?

Answer 7

TSH released from the pituitary. IT stimulates the follicular cells to endocytose colloidal thyroglobulin

Question 8

What is formed when one iodine is added to tyrosine?

Answer 8

MIT - monoiodotyrosine

Question 9

What is formed when two iodines are added to tyrosine?

Answer 9

DIT - diiodotyrosine

Question 10

What do MIT + DIT form?

Answer 10

T3

Question 11

What do DIT + DIT form?

Answer 11

T4 (thyroxine)

Question 12

What is the enzyme present on the colloidal side of the cells of the thyroid which catalyses the addition of iodine to tyrosine residues in the thyroglobulin?

Answer 12

Thyroid peroxidase

Question 13

Are T3 and/or T4 lipid soluble?

Answer 13

Both are lipid soluble

Question 14

What is the main plasma protein to which T3 and T4 bind to in the plasma?

Answer 14

Thyroxine-binding globulin

Question 15

What proportion of T3 and T4 is in the bound state with plasma proteins?

Answer 15

99.8%

Question 16

Does T3 or T4 have a longer half-life than the other? Why?

Answer 16

T4 has a longer half life than T3 - around 6 days vs 1 day

Because the affinity of thyroxine-binding hormone is higher for T4 than T3

Question 17

What are two classes of chemicals which are inhibitory to thyroid hormone secretion?

Answer 17

Glucocorticoids

Somatostatin

Question 18

Decribe the pathway of the secretion of TH

Answer 18

Tonic release in the hypothalamus triggers TRH release to the anterior pituitary, which releases TSH into the bloodstream, which acts upon the thyroid gland to stimulate T4 and T3 (T3 differs by one less iodine and is the active form of T4) and this exerts the systemic metabolic effects such as raising metabolic rate and producing heat, uncoupling oxidative metabolism in mitochondria, playing a critical role in growth and is involved in foetal brain development

Question 19

Give some causes of hyperthyroidism

Answer 19

Graves disease - antibodies are produced which bind and mimic TSH, continually activating the thyroid gland. The increased production of TH switches off TSH release from the anterior pituitary so the TSH in the plasma is very low. The thyroid gland may be 2-3x normal size due to hyperplasia

Thyroid adenoma - hormone-secreting thyroid tumour (rare)

Question 20

What are some symptoms of hyperthyroidism?

Answer 20

Heat intolerance/weight loss
Muscle weakness
Hyper excitable reflexes and psychological disturbances
Increased HR, contractile force, output -> cardiac failure

Question 21

What are some causes of hypothyroidism?

Answer 21

Hashimoto’s Disease - autoimmune attack of the thyroid gland

Deficiency in dietary iodine - only 50mg per year required but many areas of world soil has insufficient quantities

Idiopathic - no known cause, may be linked to thyroiditis

Question 22

What are symptoms of hypothyroidism?

Answer 22

Cold intolerance/weight gain
Brittle nails/thin skin
Slow speech/reflexes, fatigue
Slow heart rate/weaker pulse

Question 23

What is the manifesto that after often accompanies hypo and hyperthyroidism?

Answer 23

Goiter