1a The Thyroid Gland Flashcards
1
Q
How many lobe does a normal thyroid gland consist of?
A
2
2
Q
What is the center of the two lobes of the thyroid gland called?
A
Isthmus
3
Q
Which nerve runs close to the thyroid gland?
A
The left recurrent laryngeal nerve - supplies vocal cords
4
Q
What are the two other glands embedded in the thyroid gland called?
A
The superior and inferior parathyroid glands
5
Q
What are the parathyroid glands involved with?
A
The metabolism of calcium
6
Q
Which nerve supplies the vocal cords?
A
The recurrent laryngeal nerve
7
Q
What is the main function of the thyroid gland?
A
Metabolism regulation through the action thyroxine synthesis
8
Q
Why must surgeons take care when performing surgery on the thyroid gland?
A
The parathyroid glands nearby are involved in calcium metabolism, so there is a risk they may be damaged
9
Q
What is the function of the parathyroid glands?
A
Secrete parathyroid hormones
10
Q
How do the parathyroid glands elevate calcium levels?
A
Elevate Ca2+ levels by degrading bone and stimulating calcium release, which increases the bodies ability to absorb calcium from food.
11
Q
Where does the thyroid gland develop from?
A
The back of the throat
12
Q
What is the name of the duct which divides into two lobes to form the thyroid gland?
A
Thyroglossal duct
13
Q
What is lingual thyroid?
A
When the thyroid doesn’t develop down the thyroglossal duct, so doesnt get to the final position - results in a nodule at the back of the throat
14
Q
By what week is the thyroid gland formed in the feotus?
A
Week 7
15
Q
What is the pyramidal lobe?
A
Extension of the thyroid gland (remnant of thyroglossal duct); ~55% of individuals have it.
16
Q
Describe how T3 and T4 are produced?
A
- TSH binds to TSH receptors on the follicular cell membrane
- This triggers a cascade of inttracellular reactions - secondary messenger systems
- Activates gene expression of TG - thyroglobulin
- Also, there is active transport of iodide ions into the follicular cell and then colloid through the sodium iodide symporter
- Iodide pumps on the basal membrane means iodide ions can enter into the colloid
- Iodination of the iodide ions into iodine
- Iodine then bind to tyrosine residue on TG in another iodination reaction forming MIT (3-monoiodotyrosine)
- Two iodines binding makes DIT (3,5diiodotyrosine)
- DIT + DIT = T4 = Thyroxine
- DIT + MIT = T3
17
Q
What is the name of T3?
A
Tri-iodothyronine
18
Q
Which of T3 and T4 is more active?
A
T3
19
Q
What happens to T4
A
Gets deiodinated to form T3
20
Q
What enzyme is used for deiodination?
A
Deiodinase
21
Q
What is reverse T3?
A
an inactive form of T3 which forms when iodine is removed from the wrong place during deiodination
22
Q
How do thyroid hormones travel in the blood?
A
they bind to plasma proteins
23
Q
What are the three plasma proteins which thyroid hormones bind to?
A
Thyroid-binding globulin
Albumin
Prealbumin
24
Q
What is the full name of t4
A
tetraiodothyronine
25
Where does most circulating T3 come from?
80% from deiodination of T4 by a deiodinase enzyme (T4→ T3)
20% from directly from thyroid secretion
26
What is the half life of T3?
2 days
27
What is the half life of T4?
7-9 days
28
What percentage of T3 and T4 are bioactive components (unbound)?
T3 - 0.5%
T4 - 0.05%
29
How is the regulation of gene expression mediated?
Via thyroid hormone receptors
30
Thyroid Hormones functions?
Increased basal metabolic rate.
Increase protein, carbohydrate and fat metabolism
Have effects on CNS → Important for brain development (Maturation of CNS)
Essential for foetal growth and development.
Enhance the effects of catecholamines leading to tachycardia, glycogenolysis and lipolysis
Effects on the GI and reproductive systems
Increased heat production
31
What is cretinism?
Untreated congenital hypothryoidism - baby born with dysfunctional TH or no thyroid gland - when in womb, mother can provide foetus with thyroid hormones as they can cross the placenta, however might not able to make some of its own when out womb
32
How are TSH levels measured in new born babies?
heal prick test
33
Explain the hypothalamo-pituitary-thyroid negative-feedback system?
The hypothalamus produces TRH, which acts on the thyrotrophs to produce TSH, which acts on the thyroid to produce thyroid hormones. Once thyroid hormones are produced, this inhibits TSH and TRH so that excess thyroid hormones are not produced = negative feedback
34
What is TRH?
Thyrotrophin releasing hormone
35
Why are women more likely to have disorders of the tyroid than men?
During pregnancy, there is a rise of autoimmunity forming, which increases the predisposition to thyroid disorders
36
What is given to transiently inhibit T3 and T4? and what is this effect known as?
Large amounts of Iodide quickly inhibits T3 and T4 production - known as the Wolf-Chaikoff effect
37
What causes primary hypothyroidism?
Autoimmune thyroid diseases like Hashimotos thyroditis
38
What happens to T4 and TSH levels during hypothyroidism?
T4 levels decline and TSH increases - this is because the lack of T4 being produced stimulates more TSH in the negative feedback loop, so it accumulates
39
What is most commonly used to treat thyroid hormone deficiency?
Levothyroxine - a manufactured form of thyroxine
40
How does levothyroxine work as a drug?
It is a synthetic form of T4, therefore can be deiodinated to form T3 - bind to receptor proteins and cause metabolis effects
41
What is the most common dosage of levothyroxine?
100micrograms
42
What are some side effects of levothyroxine?
decreases bone mineral density
hyperthyroidism
heart palpitations
abdominal pains
weight loss
43
What is liothyronine?
manufactured form of T3 - can be used to treat hypothyroidism however is generally not preferred as a treatment option
44
Why is liothyronine not preferred over levothyroxine?
Liothyronine is a synthetic derivative of T3, which is pointless giving as there is already enough T3 as the T4 that is made is already deionised into T3
45
What is combined thyroid hormone replacement?
When T3 and T4 are given together
46
What are the side effects of combined thyroid hormone replacement?
Toxicity - palpitations, tremor, anxiety
47
What is the most extreme presentation of patients with hypothyroidism?
Myxoedema Coma
48
What are some features of a Myxoedema coma?
Puffy Face
Thinning hair
Thickened skin
Hypertrophy of toenails
49
What is hyperthyroidism?
When there is too much thyroxine
50
What happens to TSH levels in hyperthyroidism?
TSH levels drop
51
What causes hyperthyroidism?
Toxic multinodular goitre
Solitary toxic nodule
Grave's disease
52
What is a toxic multinodular goitre
one or more nodules (typically benign growths) in the thyroid gland that make thyroid hormone without responding to the signal to keep thyroid hormone balanced.
53
What is Graves disease?
When the antibodies bind and stimulate TSH receptors in the thyroid leading to an overproduction - continual and autonomous stimulation
54
What impact does Graves disease have on the eye?
Can cause exophthalmos - this is due to antibodies binding to the eye muscles causing bulging
55
What impact does Graves disease have on the thyroid?
causes diffuse and smooth enlargement of the thyroid gland
56
What is pretibial myxoedema?
swollen ankles antibodies stimulate the growth of soft tissue of shins
57
What are some clinical presentations of hyperthyroidism?
nervousness, weight loss, warm sticky skin, muscle cramps and weakness, enlarged thyroid gland, hair loss, tremors
58
What is TBG?
Thyroxine-Binding Globulin
59
What regime is used to treat hyperthyroidism?
The block and replace therapy
60
How does block and replace work as a treatment for hyperthyroidism?
Block the production of thyroid hormones through radiotherapy or surgery, and then give the patient levothyroxine to replace the hormones lost
61
What is pretibial myxoedema?
When there is a non-piting swelling of the shins cause by antibodies binding to the tissue and promoting growth
62
What causes exophthalamos?
When antibodies bind to the the muscles behind the eyes, causing t eyes to bulge out their sockets
63
Why are TSH levels measured in a heel prick test for infants to check for cretinism?
TSH levels are a more accurate marker of thyroid disorders as a small change in T3 or T4 can drastically impact TSH
64
What is the Wolf-Chaikoff Effect?
Where large amounts of iodine transiently inhibit T3 and T4 production - this process occurs quickly
65
What might patients be given to lover T3 and T4 levels?
Potassium iodide - this is because of the Wolk-Chaikoff effect, where large amount of iodide ions transiently inhibits T3 and T4 production
66
How is the regulation of gene expression by T3 and T4 mediated?
Through thyroid hormone receptors - they are DNA binding transcription factors that function as molecular switch in response to a ligand - they can activate or repress gene transcription
67
Describe the embryology of the thyroid gland?
1. Midline out-pouches from the floor of the pharynx
2. This leads to the development of the thyroglossal duct
3. The thyroglossal duct then splits into two lobes
4. Duct disappears leaving the foramen caecum
5. Thyroid gland then dissapears
68
What is the colloid?
Stores of thyroglobulin which is a glycoprotein that the hormone is bound to - stores thyroid hormones
69
What makes up a thyroid follicle?
Cavities surrounded by secretory cells
70
How are thyroid hormones secreted?
Hormone is reabsorbed from the cavity (containing colloid) into the surrounding follicular cell and then released into the surrounding interstitial spaces
71
What is a periorbital oedema? Hypo or Hyper
swelling of the eye, Hypo
72
What is a myxedma coma?Hypo or Hyper?
Clinical condition where patient has swelling, hypothermia and other issues. hypo
73
How many parathryid galnds are there?
4 (two pairs of two on either side)
74
Is a smooth goitre found in hypo or hyper?
Hypothryoidism
Can be hyper if it turns into a toxic multinodule goitre
75
What three ways is T3 and T4 production regulated?
- -ve feedback
- Somatostatin
- Potassium iodide
76
What 3 things does TSH do?
- increase TG (thyroglobulin)
- Increase TPO (thyroid peroxidase)
- Increase active transport of iodide ions into follicular cells via sodium-iodide symporter
77
Where are iodide ions actively pumped into
Follicular cells
78
What enzyme helps with iodination
Thyroid Peroxidase with hydrogen peroxide
79
Where does the iodine bind to on the TG
tyrosine residues
80
How are T3 and T4 cleaved from TG?
Lysosomes
81
What is a thyroid cyst?
thyroglossal duct doesn't disappear
