The Thyroid gland and Iodothyronines

Question 1

How many lobes in the thyroid gland?

Answer 1

2

Left and Right

Question 2

What does thyroid tissue consist of?

Answer 2

Follicles which consist of a central colloid, surrounded by follicular cells surrounded by parafollicular cells

Question 3

What is between the 2 lobes of the thyroid?

Answer 3

Isthmus

Question 4

Where is the thyroid?

Answer 4

Infront of the trachea

Question 5

What is the thyroid shaped like?

Answer 5

A butterfly

Question 6

TSH secreted from anterior pituitary stimulates Thyroid gland

Answer 6

TSH binds to TSH receptor on follicular cells, stimulating various processes

Question 7

What processes are stimulated by the binding of TSH to thyroid follicular cells?

Answer 7

Uptake of iodide
Synthesis of Thyroglobulin
Iodination of thyroglobulin
Coupling reaction

Question 8

How and why is iodide taken in by the thyroid?

Answer 8

Need iodide for thyroid hormones
Taken up from blood through sodium-iodide symporters
Iodide transported into colloid

Question 9

What is thyroglobulin?

Answer 9

Protein made of AAs

Question 10

What is iodination of Thyroglobulin?

Answer 10

Add iodide to Tyrosine residues in Thyroglobulin

Question 11

What catalyses iodination of Thyroglobulin?

Answer 11

Thyroperoxidase (TPO)

Question 12

What can be produced from iodination of Thyroglobulin?

Answer 12

+1 Iodide = Mono-iodo-tyrosine (MIT)

+2 Iodides = Di-iodo-tyrosine (DIT)

Question 13

What is produced in a coupling reaction of MIT + DIT?

Answer 13

T3

Question 14

What is produced in a coupling reaction of DIT + DIT?

Answer 14

T4

Question 15

After production in the colloid, where are T3 and T4 taken?

Answer 15

Back into follicular cells

Secreted into blood when required

Question 16

What is the predominant hormone secreted by the Thyroid?

Answer 16

T4

Thyroxine

Question 17

Iodothyronines

Answer 17

Thyroid hormones

Question 18

How are iodothyronines mainly transported?

Answer 18

Bound to plasma proteins in blood

Question 19

What are the proportions of binding of iodothyronines?

Answer 19

Thyroid binding globulin (TBG)= 70-80%
Albumin= 10-15%
Prealbumin= Small %

Question 20

Percentages of unbound iodothyronines

Answer 20

0. 05% T4

0. 5% T3

Question 21

How is T3 different to T4?

Answer 21

T3 is the bioactive form of Thyroid hormone

Question 22

What is formed from de-iodination of T4?

Answer 22

T3

Occurs in target tissues

Question 23

A little T3 is made in Thyroid

Answer 23

Majority of T3 made in peripheral tissue from de-iodination of T4

Question 24

T4 can be de-iodinated in a different position

Answer 24

Produces reverse T3

inactive form

Question 25

Mechanism for main action of T3

Answer 25

T3 binds to Thyroid hormone receptor | Which stimulates transcription of genes, leading to protein synthesis (in nucleus)

Question 26

T3 May also have non-nuclear actions

Answer 26

Bind to ion channels on cell surface membrane

Question 27

Thyroid hormone effects

Answer 27

Fetal growth and development | Basal metabolic rate

Question 28

Congenital hypothyroidism

Answer 28

=Cretinism | So TH & TSH measured in heel prick test

Question 29

Hyperthyroidism

Answer 29

Increases basal metabolic rate (protein, carbs, fat) Potentiates actions of catecholamines- Tachycardia Effects on GI, CNS, Reproductive system

Question 30

What effects does hyperthyroidism on GI, CNS and Reproductive system?

Answer 30

GI= Diarrhoea CNS= Agitation Reproductive system= Irregular periods

Question 31

Latent periods of thyroid hormones

Answer 31

``` T3= 12 hours T4= 72 hours ```

Question 32

Half-lives of thyroid hormones

Answer 32

``` T3= 2 days T4= 7-9 days ```

Question 33

Hyperthyroidism TSH levels

Answer 33

Expect low levels TSH

Question 34

Hypothyroidism TSH levels

Answer 34

Expect high levels TSH

Question 35

What does TRH stimulate?

Answer 35

TSH

Question 36

What does TSH stimulate?

Answer 36

Production of T3 and T4

Question 37

What is the negative feedback loop in Thyroid hormone production?

Answer 37

T3 and T4 inhibit production of TRH and TSH

Question 38

What is the Wolff-Chaikoff effect?

Answer 38

Iodide itself may inhibit Thyroid hormone release

Question 39

What hormone may stimulate TRH production?

Answer 39

Oestrogen

Question 40

What may suppress TRH production?

Answer 40

Glucocorticoids