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Metabolism, Endocrinology & Haematolgy > L18: Thyroid > Flashcards

L18: Thyroid Flashcards

(41 cards)

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1
Q

Describe the structure and location of the thyroid

A

Butterfly/bow shaped about 2-3cm wide; two lobes joined by isthmus.

Lies against front of lower larynx and uppper trachea below thyroid cartilage (adams apple). The isthmus extends from 2nd to 3rd rings of trachea

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2
Q

Is the thyroid usually visible?

A

No, only when enlarged (goitre)

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3
Q

Which 2 nerves are in close proximity to the thyroid and why during surgery must you be careful of these nerves?

A
  • Laryngeal & external branch of superior laryngeal nerves
  • They supply larynx and are involved in speech
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4
Q

How many arteries supply the thyroid?

How many veins drain the thyroid?

A

3 arteries

3 veins

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5
Q

Where are the parathyroroid gland and how many are there?

A

4 on dorsal surface of thyroid

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6
Q

Thyroid is the first gland to develop; true or false

A

True, begins at 3-4 weeks gestation

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7
Q

Describe the 5 steps of thyroid formation

A
  1. Epithelial proliferation in floor of pharynx at base of tongue
  2. Descends as diverticulum through thyroglossal duct
  3. Migrates downwards passing in front of hyoid bone (remains connected with tongue by thyroglossal duct)
  4. Thyroglossal duct degenerates
  5. Detached thyroid continues to move to final position over following 2 weeks
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8
Q

Descibe the structure of thyroid tissue; include cell types and the arrangement of these cells

A
  • 2 major cell types: follicular and parafollicular
  • Follicular arranged in functional units called follicles; follicles are spherical and lined with follicular cells. Lumen of follicle contains colloid which is rich in thyroglobulin
  • Parafollicular cells found within connective tissue that separates follicles
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9
Q

What does each of the following cells produce?

  • Thyroid follicular cells
  • Thyroid parafollicular cells
A
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10
Q

State the main cell type in parathyroid tissue and what they produce

A
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11
Q

Describe the structure of T3 and T4 produced by thyroid

A

Two tyrosines linked together with iodine at 3 or 4 positions on the aromatic ring

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12
Q

State the full names of T3 and T4

A
  • T3= triiodothyronine
  • T4= tetraiodothyronine (thyroxine))
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13
Q

Describe the synthesis and secretion of T3 & T4

A
  1. Iodide transported into follicular cells against concentration gradient via sodium-iodide transporter
  2. Thyroglobulin synthesised in folliculr cells
  3. Thyroglobulin secreted, via exocytosis, into lumen of follicle
  4. Oxidation of iodide to produce iodinating species
  5. Iodination of side chains of tyrosine residues in thryoglobulin to form MIT and DIT
  6. Coupling of MIT or DIT with DIT forms T3 or T4
  7. T3 & T4 stored in lumen as part of thyroglobulin molecule
  8. Thryoglobulin taken into follicular cells via endocytosis
  9. Proteolytic clevage of thyroglobulin to release T3 and T4
  10. T3 and T4 diffuse from epithelial cells into circulation
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14
Q

Thyroid peroxidase is a membrane bound enzyme that regulates 3 reactions involvign idodine and production of thyroid hormones; what reactions does it regulate

A
  • Oxidation of iodide to iodine (requires hydrogen peroxide)
  • Addition of iodine to tyrosine acceptor residues on thyroglobulin
  • Coupling of MIT or DIT to DIT to generate thyroid hormones within thyroglobulin protein
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15
Q

What is the ratio of T3:T4 synthesis?

A

1:10

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16
Q

How and where is T4 converted into T3? Why is it important?

What happens is wrong iodide is removed?

A

Removal of 5’ iodide in liver and kidneys. Importanat in regulating amount of active T3 as T3 has a higher activity

If 3’ iodide removed will form inactive reverse T3 which will bind to thyroid receptors without stimulating them (ANTAGONIST)

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17
Q

Briefly describe the absorption of iodine

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A
  1. Iodine obtained from diet
  2. Reduced to iodide in small intestine before absorption
  3. Iodide travels to thyroid epithelial/follicular cells via blood
  4. Taken up via sodium-iodide symporter (2Na for one Iodide)
18
Q

State some common sources of iodine

Where is most of the iodine in our body?

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A

Most of iodine in our body is in the thyroid. Sources include:

  • Dairy products
  • Grains
  • Meat
  • Vegetables
  • Eggs
19
Q

Most of T4 is converted to T3 in liver and kidneys; true or false

20
Q

How are T3 and T4 transported in blood?

Study These Flashcards
A

Hyrrophobic so must be bound to thyroxine-binding-globulin, pre-albumin or albumin

21
Q

What percentage of circulating T3 is derived from T4?

22
Q

T3 is 3x more biologically active than T4; true or false

Study These Flashcards
A

False- 4x more biologically active

23
Q

Thyroid hormones effect virtually every cell in body; what do they have an effect on?

Study These Flashcards
A
  • Cellular differentiation and development
  • Metabolic pathways
24
Q

Describe the negative feedback control of thyroid hormone secretion

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A
  • Hypothalamus secretes TRH (thyrotropin-releasing hormone) from dorsomedial nucleus
  • It secretes TRH in response to ciruclating levels of thyroid hormone, stress and temperature
  • TRH travels via hypothalamo-hypophyseal portal system to anterior pituitary
  • Stimulates secretion of TSH (thyroid stimulating hormone) from thyrotropes anterior pituitary
  • TSH travelsin blood to affect follicular cells of thyroid
25
Describe the structure of TSH Describe the 'pattern' of it's release
* Glycoprotein hormone * Two covalently linked subunits (alpha & beta; *alpha present in FSH and LH but beta subunit gives unique biological activity)* * Released in low amplitude pulses in dinural rhythm *(high levels at night, decrease early morning)* * Interacts with surface receptors on follicle cells
26
State some examples of what TSH triggers when it binds to receptors on follicular cells in thyroid
* Iodide uptake * Iodide oxidation * Thyroglobulin syntheiss * Thyroglobulin oxidation * Colloid pinocytosis into cell * Proteolysis of thyroglobulin * Cell metabolism and growth
27
Describe the two pathways, including the G proteins, TSH binding to TSH receptor can stimulate
28
Alongside stimulting gland to produce and release more hormones, state 3 effects of TSH on thyroid gland
* Increased vascularity * Increase in size (hypertrophy) * Increase in cell number (hyperplasia) of follicular cells ... leads to goitre
29
Why does T3 have a shorter half life than T4?
T3 has lower affinity for transport proteins than T4 hence more of it is free so it has half life of ~2 days whereas T4 half life ~8 days
30
Explain why there is an increased total (bound and unbound) amount of thyroid hormones in blood during pregnancy
* Oestrogens increase synthesis of thyroxine-binding globulin in pregnancy * Decrease amount of free T3 and T4 * Only free hormones can do negative feedback hence body sees this a decrease in levels so stimulates pathway to increase TSH, TRH, T3 and T4 * Amount of free T3 and T4 returns to normal but total amount increases
31
State some general actions of thyroid hormones
* **Increase in metabolic rate and heat production**: *except brain, spleen and testes. Increases number and size of mitochondria and stimulates synthesis of enzymes in respiratory chain* * **Stimulation of metabolic pathways:** *generally catabolic: lipolysis, b-oxidation fatty acids, increase gluconeogenesis and glycogenolysis* * **Sympathomimetic effects**: *increase response of target cells to catecholamines by increasing receptor number*
32
Describe the specific effects of thyroid hormones on: * CVS * Nervous system * GI tract * Ovulation
* GI: increase motility * Ovulation: ovulation fails to occur in absence of thyroid hormones as they have permissive role in actions of FSH and LH
33
Describe the actions of thyroid hormones at the target cells; describe how the hormones exert their effects
Nuclear receptors function as hormone activated transcription factors
34
What is goitre and when does it occur?
Enlargement of thyroid gland which may accompany hypo- or hyper- thyroidism; develops when thyroid gland overstimulated
35
What is hypothyroidism? State causes, symptoms and hormone levels
Too little of physiologically active thyroid hormone
36
Describe the possible complications of hypothyroidsim in infants and adults
37
What is Hashimoto's disease? How is it treated?
* Autoimmune disease resulting in **destruction of thyroid follicles** leading to undersecretion of thyroid hormones or **production of an antibody that blocks TSH receptor** on follicle preventing it from responding to TSH * **Oral thyroxine** *(give T4 as it has longer half life)*
38
What is hyperthyroidism? State causes, symptoms and hormone levels
39
What is Grave's disease? How is it treated?
* **Autoimmune** disease resulting in hyperthyroidism caused by **production of TSI** (thyroid stimulating immunoglobulin) which stimultes TSH receptors on follicle cells resulting in increased T3 and T4 production and release. * Treatment: **carbimazole** (pro drug converted into methimazole which prevents thyroid peroxidase from coupling & iodinating tyrosine residues)
40
Read this to understand how hormone levels vary dependent on cause
-
41
What is thyroid scintigraphy and what is it used for?
Technetium-99m used for isotope scanninf with gamma camera to detect size, funcion and structure of thyroid. It is suitable as it has a half life of ~1day. ## Footnote *\*Also used for bone scan, myocardial perfusion imaging, brain imaging*

Metabolism, Endocrinology & Haematolgy (20 decks)

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