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Thyroid gland Flashcards

1
Q

How many lobes form the thyroid gland?

A

2

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2
Q

Which structure does the thyroid gland wrap?

A

Trachea

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3
Q

Which structure connected the two thyroid lobes?

A

Isthmus

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4
Q

Which hormone is predominantly secreted from the thyroid gland?

A

Thyroxine

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5
Q

Which two arteries supply the thyroid gland?

A

Superior & inferior thyroid arteries

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6
Q

Which nerve supplies the thyroid gland and what major nerve does it branch off of?

A

Recurrent laryngeal nerve

Branch of Vagus nerve

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7
Q

Which cells form the epithelia of the thyroid gland?

A

Follicular cells

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8
Q

What is the mucous extracellular fluid that fills the thyroid follicles?

A

Colloid

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9
Q

Which cells secrete calcitonin?

A

Parafollicular cells

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10
Q

Describe the anatomical position of the parathyroid glands?

A

Bilateral structures (superior and inferior), are embedded within the thyroid

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11
Q

What is the purpose of the parathyroid gland?

A

Secretion of parathyroid hormone. PTH elevates Ca2+ levels by degrading bone and stimulating calcium release

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12
Q

What are the associated risks with thyroidectomy?

A

Parathyroid glands can be impacted, inadvertently interfering with calcium levels.

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13
Q

Which growth factor is involved with the development of the thyroid gland?

A

Fibroblast growth factor signalling pathways

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14
Q

Where does the thyroid gland originate from embrylogically?

A

Base of the tongue

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15
Q

Which hormone is released by thyrotroph cells and how does it activate thyroid hormone production?

A

Thyroid-stimulating hormone binds to target TSH receptors on cell-surface membrane of follicular cells

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16
Q

What action occurs during TSH receptor activation?

A

Active transport of iodide into the follicular cell via the sodium-iodide symporter

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17
Q

How is the sodium gradient maintained?

A

Sodium potassium ATPase

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18
Q

Which surface are the Sodium/iodide pumps located on thyroid follicular cells?

A

Basal membrane

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19
Q

Which protein pump, pumps iodide from the follicle into the colloid, via the apical membrane?

A

Pendrin pumps

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20
Q

Where is thyroglobulin made?

A

Within follicular ribosomes and stored into secretory vesicles

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21
Q

Which amino-acid residues are associated with thyroglobulin molecules?

A

Tyrosine residues

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22
Q

Where is thyroglobulin secreted into?

A

Secreted into the colloid

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23
Q

What is thyroglobulin?

A

Prohormone for thyroid hormone

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24
Q

How is iodide activated in the production of thyroid hormones?

A

Thyroid peroxidase

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25
What is the single iodinated form of thyroglobulin?
3-monoiodothyronine (3-MIT)
26
Where does iodination occur **on** thyroglobulin molecules?
Tyrosine residues
27
Which molecule is formed upon the secondary iodination of MIT?
3,5-Diiodotyrosine (DIT)
28
How is T3 formed?
Coupling of MIT and DIT
29
How is T4 formed?
Coupling of two DIT molecules (thyroxine)
30
What is the fate of iodinated thyroglobulin?
Endocytosis by the apical membrane into the follicular cell, subjected to proteolysis
31
What is the purpose of cleavage of T3 and T4?
Removes thyroglobulin from t3/t4 to produce the actual hormones
32
Which hormone is the active thyroid hormone?
T3
33
Which thyroid hormone is the major product?
T4
34
Which enzyme deiodinates T4 into t3?
Iodothyronine deiodinases
35
What is reverse T3?
Biologically inactive molecule formed by the removal of the wrong Iodine group
36
Under starvation circumstances, what is the fate of t4?
Convered into rT3, rather then T3
37
What is the main thyroxine transportation hormone?
Thyroxine-binding globulin (TBG)
38
What are the other thyroxine transportation hormones(Apart from TBG)?
Albumin | Prealbumin
39
Describe the binding of albumin with T3/4?
Weak/loose
40
Which thyroxine hormone predominantly is associated with prealbumin?
T4
41
What is the circulating period for T3?
12 hours
42
What is the circulating time for t4?
72 hours
43
What are the sources of T3?
80% deiodination of T4 20% MIT+DIT coupling in thyroid
44
Why can iodothyronine enter cells?
Lipid soluble, thus can interact with intracellular receptors
45
Which thyroid hormone receptors bind onto iodothyronines?
T3 response elements (TREs)
46
Which receptors do heterodimer thyroid hormones receptors bind to?
Retinoid X receptor
47
What effect does iodotyronines have on cells?
Genomic action
48
What is the main action of iodothyronines?
Increases basal metabolic rate Important in calorigeneis, Temperature regulation Increased protein, fat & carbohydrate metabolism.
49
How are thyroid levels measured?
Using a heel-prick test
50
What are the main actions of iodothyronines?
Enhances the effects of catecholamines,leading to tachycardia, glycogenolysis, and lipolysis Interaction with endocrine systems Have effects on the CNS Increase Vitamin C synthesis
51
What effect does hypothyroidism have on retinal?
Retinal accumulation, leads to yellow skin
52
Which hormones increase iodothyronine production?
Thyroid releasing/stimulating hormones and oestrogen
53
What effect does elevated levels of T3/T4 have?
Negative feedback effect, directly acting on the adenohypophysis to inhibit the secretion of TRH, and indirectly on the hypothalamus
54
What effect does somatostatin have on the anterior pituitary?
Inhibits the release of growth hormone
55
What effects do glucocorticoids have on thyrotrophs?
Inhibitory effect-Decrease blood TSH conc
56
What is the Wolff-Chaikoff effect?
Inhibits the production of iodothyronines, through inhibition of TPO. **Due to increases inorganic iodide**
57
What is primary hypothyroidism?
Autoimmune stimulated damage to the thyroid gland subsequently reduced iodothyronine productions, hence thyroxine levels decline.
58
Why are TSH levels elevated due to hypothyroidism?
TSH levels elevate due to reduced inhibitory effect from T3/T4 (Negative feedback loop is disrupted); eventually TSH levels decline as resources are exhausted.
59
Which autoimmune thyroid disease is associated with hypothyroidism?
Hashimoto's thyroiditis
60
What are the symptoms of primary hypothyroidism? (6)
Deepening voice, depression and tiredness Cold intolerance (Calorigenesis effects of secondary heat production is reduced) Weight gain Constipation Bradycardia Eventual myxoedema coma
61
Why is there weight gain with primary hypothyroidism?
Reduced basal metabolic rate of tissues and reduced lipolysis
62
what effects does T3 and T4 have on cells (genomic action)?
T4 and T3 bind to thyroid receptor proteins (TREs, and RXRs) in the cell nucleus and cause metabolic effects through transcription activation Levothyroxine is a chiral molecule in the L-form. Thyroxine supplements primarily replace the reduced level of iodothyronines.
63
Which therapy is the most effective for hypothyroidism?
T4/T3 therapy
64
What are the implications of thyroxine supplements?
Tachycardia, potential heart attack; weight loss; headaches and hyperthyroidism.
65
What is hyperthyroidism?
An overactivity of the thyroid gland, resulting in an elevated production of thyroxine, and subsequently reducing TSH levels.
66
What are the main features of hyperthyroidism?
Overproduction of thyroxine; raised basal metabolic rate; raised temperature; burn up calories and weight loss; tachycardia.
67
What are the main causes of hyperthyroidism?
**Graves’ disease** – whole gland smoothly enlarged and whole gland overactive **Toxic multinodular** goitre **Solitary toxic nodule**
68
Describe the pathophysiology of Graves disease:
Whole gland smoothly enlarged and the entirely of the gland is overactive. Autoimmune condition attributed towards increased thyroid-stimulating immunoglobulin (TSIs) levels that encourages the thyroid gland to hypertrophy, and synthesising increased amounts of thyroid hormone than required. TSIs bind to thyroid cell receptors, ’docking stations’ for thyroid-stimulating hormone. The TSI behaves as an agonist, stimulating follicular cells and thyroid follicles to synthesis iodothyronines. (Type 2 hypersensitivity)
69
What is a toxic multinodular goitre?
Excess production of thyroid hormones from functionally autonomous thyroid nodules which do not require stimulation from TSH.
70
What is a solitary toxic nodule?
A toxic adenoma, whereby a single nodule undergoes hypertrophy and produces excess thyroid hormones.
71
What is an exophthalmos?
Immunoglobulins bind to muscles behind the eye, which project the eye forwards and cause proptosis.
72
What are the clinical features of hyperthyroidism?
Myopathy; mood swings; feeling hot; diarrhoea; increased appetite despite weight loss; tremor of hands; palpitations; sore eyes and goitre.
73
What are the symptoms of hyperthryroidism?
Neuro: Insomnia, nervousness, depression CV:Tach, +BP Hair:Broken hair/hair loss Endo: Goitre Gastro:Weight loss, Fragile fingernails, shaking hands
74
What are the symptoms of hypothyroidism?
Neuro:Fatigue, Memory impairment Gastro:Weight gain, constipation Repro:Low libido, diminished fertility and potency Skin: Dry and rough Endo: Goitre Swelling of face, eye socket and deep, rough voice
75
What molecule is produced when thyroid peroxidase activates iodine
H2O2(Hydrogen peroxide)
76
What are the functions of thyroid peroxidase?
Activation of iodine Stimulation of coupling reactions
77
What are the half lives of T3 and T4?
T4: 7-9 days T3: ~2 days
78
What duct forms alongside the thyroid gland and what foramen does it leave
The thyroglossal duct leaving the foramen caecum in the mouth
79
By what time has the thyroid gland finished development
Week 7
80
What 5 congenital abnormalities can the thyroid develop?
Thyroid agenesis Lingular thyroid Retrosternal goitre Thyroglossal cyst Thyroglossal fistula
81
In thyroid: Top>Bottom Follicular cells Colloid
82
How are iodothyronines transported in the blood and in what percentages?
Mostly bound to plasma proteins a) thyroxine-binding globulin: TBG (70-80%) b) albumin (10-15%) c) prealbumin (aka transthyretin)
83
What is released by the follicular nucleus after TSH receptor activation and why does said molecule do?
Thyroglobulin, binds to activated iodine as well as assists in coupling reactions
84
Parafollicular cells
85
How are t3/t4 levels controlled?
Negative feedback into the anterior pituitary releasing TSH and hypothalamus releasing TRH
86
What are the effects of thyroid hormone?
**Effects on growth, CNS, GI, Reproductive systems ↑Basal metabolic rate Protein, carbohydrate & fat metabolism Potentiate actions of catecholamines (e.g. tachycardia, lipolysis)**
87
What occurs to T3 and T4 when entering a cell
T4 deiodinated into T3, T3 binds to receptor in nucleus causing altered gene expression
88
Whole thyroid T4/T3 production
89
Where are the Na+/I- pumps found on follicular cells?
Basolateral membrane

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