Thyroid Metabolic Hormones

Question 1

What are the 2 major metabolic hormones secreted by the thryoid?

Answer 1

1. Thyroxine (T4)
2. Triiodothyronine (T3)

Also secretes calcintonin

Question 2

Describes the steps in thyroid hormone synthesis and excretion.

Answer 2

1. Iodide trapping. Na/Iodide symporter on the basal membrane
2. oxidation to Iodine via peroxidase/ hydrogen peroxide on the apical border
3. synthesis of thyroglobulin by Golgi, secreted into the colloid
4. Organification, Iodination and coupling –> binds to the tyrosine within the thyroglobulin to form monoiodotyrosine and diioiodotyrosine
5. DIT x 2 = T4, DIT + MIT = T3
6. pintocytosis back in the follicular cell
7. fusion with lysosome –> protease reaction –> T4, T3, released into blood stream via exocytosis

MIT and DIT are deiodinated in the follicle by deiodinase, free iodine is reused

Question 3

What is the most common form of thyroid hormone released from the thyroid?

Answer 3

T4

Question 4

Which form of the thyroid hormone is more metabolically acitve?

Answer 4

T3, protein binding is not as tight as T4 and has higher affinity to receptors

Question 5

What’s the common substrate bounded to thyroglobunlin?

Answer 5

MIT and DIT (2/3),

Question 6

How is thyroid hormone transported in the bloodstream?

Answer 6

The majority are bound to protein, esp thyroxine-binding globulin, also albumin
- 99.9% of T4 is bound, 99% of T3 is bound
- only free thyroid hormone is taken up by the tissues

Question 7

Does alteration of the thyroxine-binding globulin influence free hormone leve?

Answer 7

No
negative feedback loop

Question 8

What are some physiological function of the thyroid hormone?

Answer 8

Most T4 will be converted to T3 in the target cells.
- T3 bind with nuclear receptors
- influence transcription of genes

Question 9

How does thyroid hormone influence cellular metabolic activity?

Answer 9

• increase oxygen consumption
• increase heat
• increase # and size of mitochondria
• increases key respiratory enzyme activities
• increase Na/K ATPase activity –> increases heat

Question 10

What are the effects of thyroid hormones on growth?

Answer 10

• required for normal growth and development

Question 11

An increase in metabolic rate due to thyroid hormone can lead to:

Answer 11

• thermogenesis/ sweating
• increased rate and depth of respiration
• increased cardiac output: increase in metabolism leads to local vasodilation
• increased pulse pressure but no mean arterial pressure
• increased utilization of substrate for energy (lipid, carbohydrate, protein), decreasing circulating levels of cholesterol
• increased requirements for vitamins
• increased gastric motility

Question 12

How is thyroid hormone regulated?

Answer 12

• TSH = primary controller
• binding of TSH to cell membrane –> activates adenylyl cyclase –> cAMP
• immediate effect is to promote endocytosis of thyroglobulins, release of T3/T4 into the bloodstream
• TSH also stimulates steps in thyroid hormone synthesis

Question 13

What is the best known stimuli for TSH secretion?

Answer 13

cold!