Physiology 5 - Thyroid

Question 1

List the cell types of the thyroid?

Answer 1

• C (clear) cells (Parafollicular cells)
• Follicular Cells

Question 2

What do C cells do?

Answer 2

Release calcitonin

Question 3

Describe the structure of follicular cells?

Answer 3

Cuboidal cells that make the wall of thyroid follicles, containing colloid

Question 4

What do Follicular cells do?

Answer 4

They produce Thyroglobulin (precursor to thyroid hormones) and the enzymes that make them.

They also concentrate iodide into the colloid for use in making thyroid hormones

Question 5

How do follicular cells concentrate iodide in the colloid?

Answer 5

Recieve it from the plasma through a Na+/I- symporter using Na+ conc. gradient. Pump it through to colloid through Pendrin Transporters

Question 6

What is thyroglobulin?

Answer 6

Glycoprotein containing many tyrosine residues used to make thyroid hormones

Question 7

What enzyme catalyses thyroid hormone synthesis?

Answer 7

Thyroid Peroxidase

Question 8

What thyroid hormones are made in the colloid?

Answer 8

T3 - Triiodothyronine T4 - Thyroxine Both are amine hormones made from tyrosine, hence they’re lipophilic

Question 9

Describe the process of Thyroid hormone synthesis in the colloid?

Answer 9

1) iodide oxidised to iodine
2) Iodine added to Tyrosine Residues in thyroglobulin

(1 iodine = MIT, 2 iodine = DIT)

3) MIT + DIT = T3 or DIT + DIT = T4

Question 10

What do MIT and DIT stand for?

Answer 10

Monoiodotyrosine Diiodotyrosine Tyrosine-iodine complexes which can be combined to form thyroid hormones

Question 11

What happens to transport the thyroid hormones out the colloid and cleave them into active forms?

Answer 11

Thyroid Stimulating Hormone triggers:

1) Endocytosis of colloid
2) Proteolytic enzymes then cut thyroglobulin to release THs
3) T3/4 diffuse through to plasma
4) most bind to plasma proteins

Question 12

What plasma proteins bind T3/4?

Answer 12

Mainly Thyroxine-binding Globulin (TBG)

Question 13

Summary of TH production:

Answer 13

Iodine combined with tyrosine residues by Tyrosine Peroxidase in the Colloid. TSH triggers cleaving and release of THs into plasma

Question 14

Whats the two main differences between T3 & T4?

Answer 14

T4 has a greater affinity for TBG, hence it binds better so has a longer half life (6 vs 1 day). T3 has a greater affinity for Thyroid Hormone Receptors so is more physiologically active

Question 15

In what form is most plasma thyroid hormone?

Answer 15

Protein bound T4

Question 16

What happens to T4 prior to action on Thyroid hormone receptors?

Answer 16

Its deiodinated to T3 by deiodinase enzymes. Either in plasma or target cells. The degree of deiodination can change based on need for Active THs

Question 17

What factors regulate Thyroid Hormones?

Answer 17

• Free Hormone (-ve feedback) - Cold/Exercise/Pregnancy

Inhibitory:

• Glucocorticoids
• Somatostatin

Question 18

How do Free Thyroid Hormones regulate themselves?

Answer 18

By -ve feedback Free T3 or T4 inhibits TSH release from adenophysis and TRH from hypothalamus

Question 19

How does cold, exercise and pregnancy affect thyroid hormones?

Answer 19

They all stimulate TRH release from the hypothalamus

Question 20

How do glucocorticoids affect thyroid hormones?

Answer 20

Inhibit TSH release & T4 deiodination

Question 21

How does somatostatin affect thyroid hormones?

Answer 21

Inhibits TSH release

Question 22

Where are T3 receptors found?

Answer 22

Nuclei

Question 23

What does T3 do?

Answer 23

• Raises metabolic rate
• Promotes Thermogenesis
• Promotes gluconeogenesis
• Promotes Proteolysis
• Promotes Lipolysis
• Stimulates GH receptor expression (permissive)
• Foetal Brain development
• Sensitizes Beta receptors to epinephrine (Permissive)