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MD4001 Week 9 > The Thyroid Gland: Control of Secretion, Effects of the Thyroid Hormones > Flashcards

The Thyroid Gland: Control of Secretion, Effects of the Thyroid Hormones Flashcards

1
Q

Where does the thyroid arise from?

A

Thyroid tissue arises in the midline at a point on the tongue later known as the foramen caecum

Epithelial cells sink downwards anterior to the hyoid and larynx

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2
Q

What connects the thyroid to the tongue?

A

Thyroglossal duct

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3
Q

What cells can be seen in a thyroid gland histology?

A

Follicles

Follicular cells

Colloid

Parafollicular C-cells

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4
Q

Function of follicles in thyroid?

A

Responsible for the production and secretion of the thyroid hormones thyroxine (T4) and triiodothyronine (T3)

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5
Q

Function of colloid in thyroid?

A

Through the action of thyroid peroxidase, thyroid hormones accumulate in colloid, on the surface of thyroid epithelial cells

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6
Q

Function of parafollicular C-cells?

A

Parafollicular cells, also called C cells, are neuroendocrine cells in the thyroid. The primary function of these cells is to secrete calcitonin

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7
Q

What pathology can develop in the thyroglossal cyst?

A

Should close during development

If does not fully close develop into thyroglossal cyst

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8
Q

Describe the hypothalamo-pituitary control of thyroid homrone release?

A

Small-bodied neurons in the arcuate nucleus and median eminence synthesize and secrete thyrotropin releasing hormone

Long portal vessels carry TRH to the anterior pituitary

TRH binds to receptors on thyrotroph causing G-protein coupled receptors which stimulates IP3 which releases calcium stores

Thes then cause a release of TSH from the thyrotroph

This TSh then travels to the follicular cell cytosol and causes release of thyroxine

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9
Q

What inhibits the release of TSH?

A

Somatostatin

Dopamine

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10
Q

What are the different thyroid hormones?

A

Thyroxine (T4)

Triiodothyronine (T3)

Reverse T3

Peptide backbone of thyroglobulin molecule

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11
Q

What happens to the thyroglobulin molecule in the follicular cell?

A

Inside lysosomes of the follicular cells, enzymes will cleave the two peptide bonds shown releasing T4

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12
Q

Where does T3 and T4 feedback to?

A

Anterior pituitary

Arcuate nucleus

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13
Q

What are the 8 steps in synthesis and secretion of T3 and T4?

A

TSh increases thea citvity of NA/I cotransporter on the basolateral membrane of the thyroid follicular cell. The result is increased iodine trapping: the ratio of follicular-cell iodine to plasma iodine increases under conditions of high TSH

Iodide leaves the cell, probably via pendrin and enters the lumen. The follicular cell also secretes thyroglobulin. Thyroid peroxidase, on the luminal surface of secretory vesicles, oxidizes I^- to I^0

Tsh also stimulates iodination of thyroglobulin in the follicular lumen

TSh stimulates the conjugation of iodinated tyrosines to form T4 and T3 linked to thyroglobulin

TSh stimulates the endocytosis of iodinated thyroglobulin into the follicular cells from thyroid colloid

TSH stimulates the proteolysis of the iodinated thyroglobulin, forming T4 and T3 in the lumen of the lysoendomesome

FInally TSH exerts a growth factor effect, stimulating hyperplasia within the thyroid gland

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14
Q

What is the function of 5’/3’ and 5/3 monodeiodinase?

A

Cleaving iodine from T4 and T3

This can activate and deactivate thyroxine in the blood

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15
Q

What are the different mechanisms of action of thyroid hormones?

A

Increased sodium/potassium pump

Gluconeogenic enzymes

Respiratory enzymes

Myosin heavy chain

Beta adrenergic receptors

Mainy others

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16
Q

How does T3 and T4 enter cells and alter gene make up?

A

Both pass from blood via diffusion or carrier-mediated transport

Monodeiodinase removes the 5’ iodine converting T4 to T3

Thes then enter nucleus and bind to thyroid hormone receptor

This then alters the thyroid response element causing a change in transcription and therefore a change in production of mRNA

17
Q

What is T3 and T4 bound to when carried in the blood?

A

TBG (thyroxin-binding protein)

18
Q

What are the physiological actions of the thyroid hormones

A

Increased basal metabolic rate

This increases oxygen consumption

Which increases heat production

19
Q

How does hyperthyroidism affect BMR?

A

Increases up to 100%

20
Q

How does hypothyroidism affect BMR?

A

Decreases to 50-60%

21
Q

What is the direct and indirect actions of T3 and T4 by mobilisation of carbohydrate, fat and protein stores?

A

Increased glucose uptake from GI tract

Increased glucose utilisation

Increased liver glycogenolysis and gluconeogenesis

Increased lipolysis in adipose tissue

Increased tissue oxidation of FFA

General increase in protein turnover

Increase in specific enzymes/ membrane proteins, hormone receptors

Permissive actions for other hormones (GH, prolactin, gonadal, adrenal steroids)

Essential for normal development and function of central and peripheral nervous system

22
Q

What is T3 better at?

A

Genetic editing

This is why T4 is cleaved to from T3 prior before dissolving into the nucleus

23
Q

Symptoms of hypothyroidism?

A

Poor mental ability

Lack of memory and initiative

24
Q

Symptoms of fetal hypothyroidism?

A

Neuronal hypoplasia

Delayed myelination

Mental retardation

25
Q

Example of different thyroid disorders?

A

Goitrogens

Pituitary tumours

Graves disease

Hashimoto’s disease

26
Q

Describe goitrogens?

A

Excess iodide, thiocyanate or perchlorate cause excess TSH secretion and hypertrophy of thyroid and hypothyroidism

27
Q

Describe pituitary tumours?

A

Excess TSH- hypertrophy of thyroid and hyperthyroidism

Lack of TSH- atrophy of thyroid and hypothyroidism

28
Q

Describe Graves disease?

A

Long acting thyroid stimulator

Hypertrophy of thyroid and hyperthyroidism

29
Q

Describe Hashimoto’s disease?

A

Autoimmune destruction of thyroid

Atrophy of thyroid and hypothyroidism

30
Q

What can thyroid hormone replacement therapy do?

A

Increase bone to match chronological age with developmental age

Same with height

Mental age, no real change

31
Q

What does thyroid hormone do to glucose?

A

By stimulating liver, can increase blood sugar level

32
Q

What causes exophthalmos?

A

The immune system attacks the muscles and fatty tissues around and behind the eye, making them swollen

MD4001 Week 9 (8 decks)

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