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Metabolism, Endocrinology And Haematology > Thyroid Gland & problems > Flashcards

Thyroid Gland & problems Flashcards

1
Q

Where is the location of the thyroid gland ? And what is the shape of the thyroid gland ?

A

Lies against and around the front of larynx and trachea.

It lies below the thyroid cartilage ( Adam’s apple )

STRUCTURE : Butterfly shaped / bow shaped gland. There are two lobes which are joined by the isthmus.

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2
Q

What is the first endocrine gland to develop during embryology ?

A

Thyroid gland

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3
Q

How does the thyroid gland form during embryological development ?

A
  • it is the first endocrine gland to form during embryological development.
  • at 3-4 weeks gestation , thyroid gland appears as an epithelial proliferation in floor of pharynx at base of tongue and takes several week to migrate to final position via the thyroglossal duct.
  • during migration , the thyroid gland remains connected to tongue by thyroglossal duct which subsequently degenerates.
  • detached thyroid then continues to its final position over following two weeks.
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4
Q

Describe the histology of thyroid tissue

A

Follicular cells arranged in spheres called thyroid follicles.

The thyroid follicles are filled with COLLOID which is considered extracellular even though it is contained within the follicle. The thyroid follicles produce thyroid hormone.

COLLOID contains a deposit of thyroglobulin.

Thyroid parafollicular cells ( larger ) release calcitonin.

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5
Q

What are the two thyroid hormones produced by thyroid gland

A

T3

T4 ( thyroxine )

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6
Q

What 2 structures form T3

A

Monoiodotyrosine + diiodotyrosine = T3

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7
Q

What 2 structures formT4 ( thyroxine )

A

Diiodotyrosine + Diiodotyrosine = T4

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8
Q

What is the function of thyroglobulin?

A
  • thyroglobulin( protein) is synthesised in the follicular cells and secreted into the colloid cells.
  • There are many tyrosine residues found on the thyroglobulin ( around 134).
  • these tyrosine residues undergo iodination. And then a coupling reaction occurs between the two iodo-tyrosine structures. Which forms either T3/T4.
  • subsequent degradation of thyroglobulin will release T3/T4.
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9
Q

What is the name of a key enzyme required for iodination of tyrosine , coupling of the two iodination-tyrosine molecules & oxidation of iodide into iodine ?

A

Thyroid peroxidase

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10
Q

What are the three functions of thyroid peroxidase ?

A
  1. Addition of iodine to tyrosine acceptor residues on the protein thyroglobulin.
  2. Coupling of MIT or DIT to generate thyroid hormones ( T3/T4) within their thyroglobulin protein.
  3. Oxidation of iodide to iodine :
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11
Q

Why is thyroid peroxidase required to oxidise iodide to iodine ?

A

Because dietary iodine is reduced to iodide before it can be absorbed into our small intestine.

The iodide is then taken up from the blood by thyroid epithelial cells which have a sodium -iodide symporter. Once iodide has entered these cells , they are then oxidised into iodine by thyroid peroxidase in order to make thyroid hormones

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12
Q

Summarise thyroid hormone synthesis

A
  1. Thyroglobulin is synthesised in the follicular cells from amino acids.
  2. This is then transported into the colloid cells which stores this protein.
  3. The thyroglobulin contains tyrosine residues.
  4. The tyrosine residues must be iodinated by iodine with the aid of thyroid peroxidase. The iodine enters the colloid cell via the Na+ iodide symporter ( 2Na+ in for every I- in) after it has been oxidised into iodine from iodide with the help of thyroid peroxidase.
  5. Thyroid peroxidase helps with the coupling of tyrosine-iodine structures to form the thyroid hormones.
  6. The follicular cells pinches the colloid into itself via pinocytosis. Contains the colloid cells into a vesicles which fuses with lysosome + they degrade the thyroglobulin protein which releases the thyroid hormones.
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13
Q

What thyroid hormone is predominantly released ?

A

90% of thyroid hormone released is T4.

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14
Q

Which hormone is more biologically active - T3 or T4 ?

A

T3 is around4 x more active than T4.

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15
Q

What percentage of circulating T3 is derived from T4 ?

A

80% of T3 is derived from T4. Most T4 is converted into T3 in the kidneys and liver.

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16
Q

What protein is used to transport T3 and T4 in the blood ?

A

Thyroxine - binding globulin.

17
Q

What influences release of thyroid hormones ( T3+T4) ?

A

Hypothalamus - thyrotropin releasing hormone ( TRH)

Anterior pituitary- TSH ( thyroid stimulating hormone )

Thyroid gland - Thyroid hormones

18
Q

How does TSH trigger the release of thyroid hormones ?

A
  1. Activates TSH receptor on the follicular cells which is a G protein coupled receptor ( both Gs & Gq)
  2. Activation of this receptor will promote : iodine uptake , iodine oxidation , thyroglobulin synthesis , colloid pinocytosis into cell , proteolysis of thyroglobulin,
19
Q

What are the general actions leg thyroid hormone ?

A
  1. Increases the basal metabolic rate and heat production - it does this by increasing the number and size of mitochondria. And stimulating the synthesis of enzymes in the respiratory chain.
  2. Stimulation of metabolic pathway : Stimulates lipolysis and beta oxidation of fatty acids. Stimulates gluconeogenesis and glycogenolysis.
  3. SYMPATHOMIMETIC EFFECTS : Increases target cell response to catecholamines by increasing expression of adrenergic receptors
20
Q

What is the effect of thyroid hormone of the CV system ?

A

Increases hearts responsiveness to catecholamines by increasing cardiac output ( because there’s an increase in heart rate and increased force of contraction)

There’s also peripheral vasodilation to carry extra heat to body surface.

21
Q

What is the effect of thyroid hormones on the nervous system ?

A

Essential for Myelination of nerves and development of neurones

22
Q

How does the thyroid hormone mediate its effect ?

A
  1. Thyroid enters cell cytoplasm from blood via thyroid hormone transport.
  2. Thyroid hormones are lipid soluble and enters nucleus and binds to thyroid hormone receptor on DNA.
  3. Binding relieves expression of gene transcription and the thyroid hormone gene is now expressed.( eg genes for Ca2+ ATPase , Na+/K+ ATPase)
  4. Allows for expression of new protein from activation of that gene which facilitates the response of the thyroid hormone.
23
Q

What does the term Goitre refer to ?

A

Enlargement of the thyroid gland.

This may accompany either Hypo/Hyperthyroidism. Hypothyroidism ( caused by lack of dietary iodine) will Defoe cause formation of goitre.

24
Q

What is Hypothyroidism ? And what are a few causes ?

A

Low levels of thyroid hormones .

  1. Inadequate supply of iodine.
  2. Failure of the thyroid gland.
  3. TSH or TRH deficiency.
  4. Radioactive iodine.
  5. Post surgery
  6. Autoimmune disease : Hashimotos
  7. Anti-thyroid drugs
25
Q

What are general symptoms of hypothyroidism?

A
  • LOW T3/T4. And elevated TSH.
  • obesity
  • lethargy
  • bradychardia
  • dry skin
  • alopecia
  • constipation
  • slow reflexes.
26
Q

What is cretinisim?

A

When there is a lack of thyroid hormones during child development.
This leads to slow pulse , muscle weakness , dwarfed stature , mental deficiency , GI disturbances

27
Q

What is myxedema?

A

A general term used to describe advanced hypothyroidism is adults.

28
Q

What is the most common form of hypothyroidism?

A

Hashimotos disease

29
Q

What is Hashimotos disease ?

A

Auto immune disease resulting in destruction of thyroid follicles. Leads to hypothyroidism.
This is more common in women than men.
PLASMA LEVELS - low T3/T4 and elevated TSH
SYMOTPMS - goitre may be present or not.
TREATMENT : Oral thyroid hormone , T4 used since half life longer.

30
Q

What is hyperthyroidism? And what are causes? And symptoms ?

A

Too much thyroid hormones.

Causes : autoimmune Graves’ disease , drugs such as amiodarone , excessive T4/3 therapy , solitary toxic adrnoma ,

SYMPTOMS : weight loss , irritability , heat intolerance , sweating warm vasodilated hands , tachycardia , fatigue , increased bowel movements , increased appetite , loss of libido , hyper reflexive , breathless, goitre , bulging eyes

31
Q

What is Graves’ disease

A

Autoimmune disease :

Where there is production of thyroid stimulating immunoglobin (TSI) which continuously stimulates thyroid hormone secretion.

This increase in thyroid hormone will feedback to P glands which will suppress the production of TSH.

SYMPTOMS - bulging eyes ( TSI reacts with fibroblasts in the eye) , goitre , increase in basal metabolic rate , excessive sweating , heart palpitations , decrease in body weight.

32
Q

How can we visualise thyroid gland ?

A

Technetium scintigraphy - technetium used for isotope scanning of thyroid gland with gamma camera. Most commonly used radioisotopes because it has a half life of 1 day. Thus , radiation exposure is low.

33
Q

What are most commonly used anti- thyroid drug ?

A

Carbimazole which is a pro drug that is converted into methimazole in body.

This prevents thyroid peroxidase from coupling and iodinating tyrosines on thyroglobulin.

34
Q

What foods contain iodine ?

A

Fish ( COD + tuna )

Seaweed
dairy eg milk , yoghurt , cheese
Breads + cereals

Metabolism, Endocrinology And Haematology (23 decks)

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