Thyroid physiology

Question 1

What is the nervous innervation to the thyroid?

Answer 1

Sympathetics using vasomotor not secretormotor.

Question 2

What is the functioning unit of the thyroid?

Answer 2

The follicular cells which are cuboidal epithelium.

Question 3

Which cells in the thyroid secrete calcitonin?

Answer 3

The parafollicular cells.

Question 4

Stored iodine is known as what?

Answer 4

Stored as iodothyronines with a reserve to last about two months.

Question 5

Describe the process of Iodine trapping.

Answer 5

Iodine trapping utilizes the 2NA/I symporter (NIS) located in the basolateral membrane to transport iodide into the follicular cells.

The gene that creates NIS is activated by TSH.

Question 6

Describe the process of organification

Answer 6

The protein known as “Pendrin” translocates the iodide into the lumen where it is immediately oxidized to iodine and attached to tyrosine-thyroglobulin.

Question 7

What action does thyroid peroxidase (TPO) have?

Answer 7

This makes MIT, DIT, T3 and T4 from organified iodide/thyroglobulin.

Question 8

When is T3 formation favored?

Answer 8

T3 formation is favored under circumstances of low iodine levels.

Question 9

What exactly is Colloid?

Answer 9

Colloid is the stored form of T3 and T4 within follicles.

Question 10

How are thyroid hormones secreted?

Answer 10

Thyroglobulin binds megalin and is endocytosed/degenerated freeing the T3 and T4 into the blood basally.

Question 11

What chemical provides feedback to the thyroid?

Answer 11

T3 formed in the anterior pit from T4 conversion via the action of Type II Deiodinase.

Question 12

What does Type II Deiodinase do?

Answer 12

Part of the regulation of TSH secretion in the Anterior pit, it converts T4 into T3.

Question 13

What are the ways in which thyroid hormone is transported in the blood?

Answer 13

Thyroixine binding globulin (TBG) - Binds T4 tighter.
Transthyretin (TTR) transports both thyroxine and retinol.
Albumin and lipoproteins
1% of all thyroxine is in free form allowing feedback.

Question 14

How is gene transcription set up in the nucleus for thyroid hormones?

Answer 14

Retinoid X receptor in the nucleus.

T3 more active for gene expression.

Question 15

What is Cretinism?

Answer 15

Too little thyroid hormone released early in life leading to mental retardation and failure of body growth.

Question 16

If hypothyroidism is occuring what should TSH levels be at?

Answer 16

TSH levels should in theory be high.

Question 17

What causes Graves disease and what symtpoms does it display?

Answer 17

Excessive thyroid hormones leading to thyrotoxicosis.

Question 18

What does Hashimotos disease cause?

Answer 18

Hashimotos disease causes Hypothyroidism “Myxedema”

Question 19

Define Thyroxine-bnding prealbumin (TBG)

Answer 19

Major binding protein for circulating thyroid hormone in which 70% is bound to this.

Question 20

Define Transhyretin

Answer 20

Additional thyroid binding protein that has the ability to bind and transfer retinol (vitamin A) as well.

Question 21

Define Thyroglobulin

Answer 21

The storage form of thyroid hormones collectively making up colloid.

Question 22

Define thyrotorpin

Answer 22

Another name for TSH stimulating synthesis of T4 (thyroxine) and T3.