The Thyroid Gland Flashcards Preview

Regulatory Physiology and Pharmacology > The Thyroid Gland > Flashcards

Flashcards in The Thyroid Gland Deck (60)
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1

Where is the thyroid gland?

wrapped around the trachea

2

How is the thyroid gland structured?

follicles lined with epithelial thyrocytes which are spherically arranged

3

What cells are in the thyroid gland?

C -cells which are parafollicular and thyrocytes

4

What does the thyroid gland secrete?

T3, T4 and calcitonin

5

What are the proportions of thyroid hormone produced?

T3-10%, reverse T3-1% and T4-90%
T3 is more potent than T4

6

How is T4 converted to T3?

a deiodinase enzyme found in peripheral tissues

7

What is the action of T3?

binds to high affinity T3Rs (TRa and TRb) associated with DNA in cell nucleus -> activate or represses gene transcription and thus regulate mRNA synthesis and protein synthesis

8

What are the features of the different types of deiodinases?

Type 1 - low affininty 10microM km, occurs in tissues with high blood flow and plasma exchange...i.e. liver, kidney and skeletal muscle, supplies circulating T3 for uptake by other tissues
Type 2 - high affinity 1nanoM km, expressed by glial cells in the CNS, provides T3 even when T4 levels are low, present in the thyrotropes of the pituitary acting as a thyroid axis sensory

9

Where does thyroid hormone biosynthesis occur?

Within the colloidal cavity

10

What does TH biosynthesis require?

thyroglobulin - secreted into follicle colloid
iodine ions - transported into follicular cells by Na-I symporter and into colloid by pendrin
Tyrosine

11

What transports I into the colloid?

pendrin

12

What is iodination?

adding iodine to thyroglobulin

13

How is thyroglobulin made into thyroid hormone after iodination?

endocytosed into vesicles and then fuse into lysosomes which degrade the tyrosine molecules into thyroid hormones

14

which enzyme catalyses the synthesis of thyroid hormones?

thyroid peroxidase

15

What processes does thyroid peroxidase catalyse?

iodination
coupling sequential reactions

16

What are the carrier proteins that thyroid hormones bind to?

thyroxine-binding globulin - major TBG
transthyretin - delivering to CNS - minor
albumin

17

How much iodine is ingested and excreted everyday?

400micrograms ingested:
150micrograms in adults
90-120micrograms in children
200micrograms in pregnant women

18

how much iodine is taken up by the thyroid gland?

70-80micrograms

19

what is the total iodide content of the average thyroid?

7500micrograms - about 70-80 micrograms is released daily

20

Where is iodide actively concentrated?

thyroid gland, salivary glands, gastric glands, lacrimal glands, mammary glands and choroid plexus

21

What can stimulate thyroid hormone secretion?

Low BMR, cold

22

What can inhibit thyroid hormone secretion?

trauma, stress, T3 - ant. pit/hypothalamus, T4 - ant. pit./hypthalamus and TSH - hypothalamus

23

What pathways does TSH stimulate at the thyrocytes?

Gs -> Ac -> PKA
Gq -> PLC -> PKC and Ca[i]

24

What does the stimulation by TSH result in?

increased iodide uptake
increased synthesis of peroxidase
increased synthesis of thyroglobulin
increased colloid uptake

25

what is the Thyroid receptor?

a nuclear receptor that regulates transcription

26

How does the thyroid receptor function?

as a heterodimer with the retinoic acid X receptor, TR-RXR binds to the thyroid response element on target gene

27

What happens with TR-RXR in the absence of TH?

represses gene transcription through recruitment of a compressor complex containing histone deacetylase (HDAC)

28

What happens with TR_RXR in the presence on TH?

coactivator complexes including histone acetyl transferase, increased acetylation and promotes transcription

29

What binds RXR?

Retinoic acid

30

How do T3 and T4 promote accelerated metabolism?

increased fat, carbohydrate and protein turnover
increase O2 consumption and heat production
regulate BMR and body temperature