The Thyroid and Iodothyronines

Question 1

Where is the Thyroid located?

Answer 1

Top of Trachea

Question 2

How many lobes has the Thyroid?

Answer 2

2

Question 3

What is the tissue that joins the two lobes of the thymus together called?

Answer 3

The Isthmus

Question 4

What is the thyroid pyramid?

Answer 4

The bit of tissue on the top of the isthmus which is present in some people (important clinically because a thyroid tumour could be in the pyramid)

Question 5

What are the three main components of the thyroid?

Answer 5

Follicular cells

Colloid: Proteinaceous substance found in the inside of follicles

Parafollicular cells: Found between the follicles and they also provide hormones

Question 6

How does iodine enter Follicular cells?

Answer 6

Iodine pumps on basolateral surface from blood

Question 7

What happens to iodine that enters follicular cells?

Answer 7

It is pumped into the colloid by Pendrin pumps, to be used in hormone production

(This is stimulated by TSH)

Question 8

What are the four effects of TSH on a follicular cell?

Answer 8

1. Stimulation of iodine pumps
2. Stimulation of nucleus to produce thyroglobulin
3. Stimulation of Thyroid Peroxidase (TPO) enzyme
4. Stimulates movement of lysozymes towards apical membrane to collect colloid to secrete into blood

Question 9

What does Thyroid peroxidase do?

(TPO)

Answer 9

TPO converts iodine into a short-lived reactive form using H₂O₂

also

TPO allows coupling reactions where T3 & T4 are formed

Question 10

What is thyroglobulin?

Where does it go once synthesised?

Answer 10

Long array of amino acids that have a certain number of Tyrosyl residues which become iodinated by reactive iodine.

Remains associated with the apical membrane

Question 11

What two products can result from the iodination of Tyrosyl residues?

Answer 11

MONOIODOTYROSINE (MIT)

and

DIIODOTYROSINE (DIT)

Question 12

What are the products of coupling reactions by TPO?

Answer 12

Triiodothyronines (T3)

and

Tetraiodothyronines (T4)

Question 13

What is Tetraiodothyronine also known as?

Answer 13

Thyroxine

Question 14

How do iodothyronines get into the blood?

Answer 14

Movement of T3/T4 bound to TG into lysozymes at the apical membrane

Enzymes remove TG, liberating the hormones which can then move out into the blood.

Question 15

How are Iodothyronines transported in the blood?

Answer 15

Bound to plasma proteins to prevent uptake by non-target cells.

Thyroxine-Binding Globulin is specific to T3/T4, but can also weakly bind to albumin

Question 16

What are the latent periods and half-lives of Thyroid hormones?

Answer 16

Latent Periods:

T3 = 12 hours

T4 = 72 hours

Biological Half Lives:

T3 = 2 days

T4 = 7-9 days

Question 17

What is the main hormone product of the thyroid, and what is the most active thyroid hormone?

What does this mean?

Answer 17

Main hormone product: T4

More bioactive hormone: T3

Therefore T4 is deiodinated in target tissues

Question 18

What is reverse T3?

Answer 18

T4 which has been deiodinated in a different position formin rT3 which is biologically inactive

Question 19

What is the main action of Iodothyronines?

Answer 19

Increases Basal Metabolic Rate (BMR)

(Except in the brain. Useful for calorigenesis which is used in temp reg)

Increase Protein, Carbohydrate, and Fat metabolism

Potentiates action of catecholamines leading to tachycardia and lipolysis - may present with palpitations

Some effects on GI/CNS/Reproductive

Question 20

What is the effect of congenital hypothyroidism?

Answer 20

If untreated within a few months, leads to cretinism.

Test for TH/TSH in heel prick test

Question 21

Describe the control of TH production:

(hypothalamo-pituitary-thyroidal axis)

Answer 21

Thyrotrophin Releasing Hormone (Hypothalamus)

→

Thyrotrophin (TSH) (adenohypophysis)

→

Iodothyronines (T3 & T4) (Thyroid gland)

Question 22

Describe the negative feedback of Iodothyronine production

Answer 22

T3/T4 directly inhibits Anterior pituitary Thyrotrophin production and indirectly inhibits Thyroid releasing hormone from hypothalamus