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Flashcards in Thyroid Gland Deck (68)
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1

The thyroid gland produces iodothyronines ____ and _____

Tetraiodothyronine (T4) aka thyroxine

Triiodothyronine (T3)

2

Peripheral conversion of iodothyronines by ____ is key

Deiodinases

3

The functional unit of the thyroid gland

Thyroid follicle

4

The thyroid follicle is lined by _____ epithelial cells and the lumen is filled with _____. Follicles also contain parafollicular C cells which secrete _____

Cuboidal; colloid; calcitonin

5

What is the main secretory product of the thyroid?

T4 (10x more than T3)

6

Colloid is composed of newly synthesized thyroid hormones attached to ____

Thyroglobulin

7

Conversion of T4 to T3 occurs in the thyroid and peripherally through the action of ______

Deiodinase

[90% is peripherally converted vs. 10% that is directly secreted]

8

What are some clinical states that are associated with reduced conversion of T4 to T3?

Fasting
Medical and surgical stress
Catabolic disease

9

_______ is often associated with elevated levels of type 1 deiodinase, contributing to elevated T3 in the blood

Hyperthyroidism

10

T4 may be converted to T3 by ____ ring deiodination (activation) by deiodinases type ___ and ___

Outer; 1; 2

11

What is the primary source of dietary iodine?

Iodized table salt

[also from dairy products and fish]

12

Synthesis of thyroid hormone involves basal to apical movement of 2 precursors ____ and _____

Thyroglobulin
Iodide

[iodide is taken up from blood; thyroglobulin is assembled from translated amino acids]

13

______ is the process of binding iodine with thyroglobulin

Organification

14

What are the important transporters in the basolateral membrane of cells synthesizing thyroid hormone?

Na/K-ATPase

Na/I symporter (NIS, 2:1 ratio)

15

What are the important transporters in the apical membrane of cells synthesizing thyroid hormone?

Pendrin — Cl/I countertransporter

Peroxidase — oxidizes iodide for combination with thyroglobulin (aka TPO)

16

After iodination by thyroid peroxidase, thyroglobulin is stored in the lumen of the follicle as colloid.

Within the colloid, thyroglobulin binds to ____, ____, and intermediates ___ and ____

T4; T3; MIT; DIT

17

When iodide availability is restricted, formation of _____ is favored

T3

18

Intracellular steps of thyroid hormone synthesis involve _______ of colloid droplet, at which time ______ cleave T4 and T3 from thyroglobulin for release into circulation

Pinocytosis; proteases

[purpose is to rapidly deiodinate MIT and DIT molecules that have been released during proteolysis of thyroglobulin. This iodide is then recycled into T4 and T3 synthesis]

19

T/F: a deficiency in intrathyroidal deiodinase mimics a dietary iodide deficiency

True

20

What are some inhibitors of NIS transporter in basolateral membrane?

Perchlorate; thiocynate

21

What is the primary inhibitor of TPO?

Propylthiouracil (PTU) - inhibits production of final thyroid products

22

The wolff-chaikoff effect results in inhibition of what step in thyroid hormone synthesis?

Organification (process of binding iodine with TG)

23

Iodine is stored as iodinated _____ of thyroglobulin

Enough hormone is stored as iodinated TG in the follicular colliod to last 2-3 mos. Each TG molecule contains 30 ____ molecules and a few ___ molecules

Tyrosine

T3; T4

24

Activity of the thyroid gland can be assessed by radioactive iodine uptake. Compare levels at 24 hrs in normal thyroid, hyperthyroid, and hypothyroid

Normal = 25% at 24 hrs

Hyperthyroid = >60% at 24 hrs

Hypothyroid = <5% at 24 hrs

25

The majority of thyroid hormone circulates bound to plasma proteins (only 1% is free). What are the main plasma proteins that bind thyroid hormones?

Thyroxin-binding protein (TBG) = 70%

Transthyretin (TTR) = 10-15%

Albumin = 15-20%

26

The main plasma protein that binds thyroid hormone is TBG. It is synthesized by the ____ and has greater affinity for ____

Liver; T4

27

Circulating TBG can be indirectly assessed with what test?

T3 resin uptake test

28

Describe T3 resin uptake test

Add labeled T3 then add resin as secondary binder

If TBG levels are already occupied, less radioactive T3 will bind and more spots will be available to secondary resin

The amount of T3 bound to resin is then measured

29

What would T3 resin uptake test results be in the case of hyperthyroid vs. hypothyroid?

Hyperthyroidism = high T3 resin uptake

hypothyroidism = low T3 resin uptake

30

What would T3 resin uptake test results be in the case of high TBG levels?

Increased T4 but decreased T3 resin uptake