The Thyroid Gland and Iodothyronines Flashcards

1
Q

Describe the structure of the thyroid gland.

A

It has two lobes joined by a narrowing called the ISTHMUS It may have a remnant of the thyroglossal duct called the PYRAMID

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

Describe the cellular arrangement of the thyroid gland.

A

The cells are arranged in follicles There are FOLLICULAR CELLS around the outside with a proteinaceous substance called COLLOID inside the follicles There are parafollicular cells between follicles

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

What three main things does TSH stimulate in the follicular cell?

A

Stimulates IODIDE CHANNELS to transport iodide from the blood into the cell at the basolateral membrane (facing the blood) and to pump out into the colloid via the pendrin pumps
Stimulates synthesis of THYROGLOBULIN Stimulates THYROID PEROXIDASE

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

what does thyroid peroxidase do?

A

In the presence of hydrogen peroxide, thyroid peroxidase can convert the iodide (which has been pumped into the colloid by the pendrin pumps) into a reactive iodine form.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

Describe the formation of T3 and T4 in the follicular cell.

A

TSH stimulates synthesis of a protein called thyroglobulin (moves to the apical surface - facing colloid) which is a long array of amino acids containing TYROSYL RESIDUES. The tyrosyl residues can be iodinated by the reactive iodine in ONE or TWO positions forming MONOIODOTYROSINE (MIT) and DIIODOTYROSINE (DIT) respectively. Continued stimulation of thyroid peroxidase will cause coupling reactions forming T3 and T4

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

At which positions are tyrosine iodinated?

A

3 and 5

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

Describe how iodothyronines are transported in the blood when it is stimulated by TSH

A

TSH then stimulates lysosomes to move towards the apical membrane and stimulates the uptake of colloid by the apical membrane.
-the colloid that has been internalised then fuses with the lysosomes and the enzymes break down the protein, liberating T3 and T4 which can move out into the blood.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

Describe how iodothyronines are transported in the blood.

A

To prevent uptake by non-target tissues, T3 and T4 are mainly transported by PLASMA PROTEINS The majority of T3 and T4 are transported by THYROXINE BINDING GLOBULIN (TBG) Some T3 and T4 is transported by ALBUMIN (binds more loosely) The BIOACTIVE component is the T3 and T4 that is UNBOUND

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

How do the latent periods and half-lives of T3 and T4 differ?

A

Latent Periods: T3 = 12 hours T4 = 72 hours Half Lives: T3 = 2 days T4 = 7-9 days
T4 is the main hormone product of the thyroid gland
T3 is the more active of the two.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

Describe the deiodination of T4.

A

T4 can be deiodinated to produce T3 which is MORE ACTIVE T4 can also be deiodinated in a different position to produce REVERSE T3 (which is INACTIVE)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

in what situations would you want to convert t4 to rt3?

A

in situation where you want reduced metabolism, e.g. starvation

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

What are the main actions of iodothyronines 1 and 2 ?

A

iodothyronines 1:

Increase basal metabolic rate Increase carbohydrate, fat and protein metabolism (in both directions: anabolic and catabolic)

iodothyronines 2:

Enhance the effects of catecholamines Interact with other endocrine systems
Have effect on CNS Increase vitamin C synthesis from retinal

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

Iodothyronines are important in growth and development. What disease is caused by a lack of iodothyronines?

A

Cretinism

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

Describe the action of iodothyronines on target cells.

A

Thyroid hormones are NOT steroid hormones but they are LIPID SOLUBLE They bind to intracellular receptors and have a genomic effect, as it has its effect in the nucleus which leads to new proteins being synthesised.
also affects membrane transport pumps, and has lots of effects in the mitochondria.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

State two hormones that increase iodothyronine production.

A

Thyroid Stimulating Hormone (adenohypophysis)

Oestrogens

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

State some factors that decrease iodothyronine production.

A

T3/T4 negative feedback (direct on adenohypophysis and indirect on hypothalamus
-Somatostatin
-Glucocorticoids
Ingestion of large amounts of inorganic iodide Thyrotrophin (autonegative feedback loop)

17
Q

how is TH levels and TSH measured?

A

in new-born infant’s heel-prick test

18
Q

explain the hypothalamo-pituitary thyroidal axis

A

neurones originating from the hypothalamus release thyrotrophin releasing hormone into the primary capillary plexus.

  • TRH passes down the portal vessels to the adenohypophysis where it will bind to membrane receptors
  • TRH will work on thyrotrophs to produce thyrotrophin
  • thyrotrophin then has lots of effects on the follicular cells in the thyroid.
19
Q

What is the clinical importance of the pyramid?

A

Thyroid tumour could be in the pyramid

20
Q

Where are thyrotrophin receptors found?

A

On serosal/basal membrane

21
Q

How is iodine pumped into the cell from the blood?

A

Via iodine pumps

22
Q

Where are thyroid hormones produced and what does this mean for iodine?

A

Produced on the apical membrane and so iodine needs to be pumped out into the colloid by pendrin pumps