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medicine > WEEK 7: 7.6 Thyroid and Parathyroid > Flashcards

WEEK 7: 7.6 Thyroid and Parathyroid Flashcards

1
Q

Where is the thyroid gland located?

A

immediately below the larynx on each side of and anterior to the trachea

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2
Q

What kind of shape is the thyroid gland?

A

a bow shape

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3
Q

What are thyroid follicles

A

spherical structures that form the functional units of the thyroid

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4
Q

What are the thyroid follicles composed of, and what is their function

A

They are composed of follicular cells, which are specialised pubertal epithelial cells that play an important role in the formation & secretion of thyroid hormones

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5
Q

What are thyroid colloid

A

gelatinous substances that contain protein called thyroid globulin

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6
Q

what are thyroid globulin molecules

A

they are effectively the protein backbone upon which the thyroid hormones are formed between the follicles, like the power follicular cells

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7
Q

what is calcitonin

A

it reduces the resorption of bone during times of high calcium demand, such as during pregnancy and breastfeeding. it plays an important role in the everyday regulation of free plasma calcium concentration

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8
Q

What stimulates the production of thyroid hormones?

A

thyroid stimulating hormone, which is secreted into the bloodstream and circulates/acts on the cells of the thyroid gland to produce thyroid hormones

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9
Q

What is secretion from the anterior pituitary gland regulated by?

A

hypothalamic hyper physio tropic hormones

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10
Q

What are the two ways in which the effects of thyroid hormones can be broadly classified?

A
  • effect on metabolic pathways- thyroid hormones boost energy metabolism in mitochondria increasing basal metabolic rate- therefore will also influence body temp
  • effect on cellular differentiation and development - hormones promote the development and differentiation of many cells including the neurons and supporting cells of the CNS
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11
Q

What does thyrotopin-releasing hormone (TRH) stimulate the release of?

A

thyroid stimulating hormone (TSH)

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12
Q

What kind of feedback loop maintains a constant supply of thyroid hormones?

A

a negative feedback loop

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13
Q

how are T3 and T4 transported in the blood?

A

by carrier proteins

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14
Q

What are non-genomic effects of TSH

A

-enhances iodide pump activity which increases iodide trapping (iodine is needed to make thyroid hormones)
-increases iodination of tyrosine which increases T3 and T4 synthesis
-increases proteolysis (breakdown) of thyroglobulin which increases release of T3 and T4

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15
Q

what are genomic effects of TSH

A

it promotes gene transcription for iodide pump, thyroglobulin, T3 and T4 synthesis enzymes, nitric oxide synthase leads to vasodilation which therefore increases blood flow to the thyroid, local growth factors lead to hyperplasia (more cells) and hypertrophy (bigger cells), hence the gland grows

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16
Q

What are non genomic effects

A

fast, direct changes without altering DNA transcription

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17
Q

What are genomic effects?

A

slower, through gene transcription and new protein production

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18
Q

How does TSH start its effect on thyroid follicular cells

A
  1. it bind to a G protein coupled receptor on the follicular cell
  2. it activates cAMP inside the cell
  3. cAMP leads to changes through 2 broad types of effects (genomic and non-genomic)
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19
Q

Where are thyroid receptors found?

A

within the nucleus of the target cells

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20
Q

What is the overall action of thyroid hormones on the cardiovascular system?

A

improving cardiac output by increasing excitability and contractility of cardiac muscle, by increasing the no of ca na and k+ pumps and increasing the expression of beta adrenergic receptors, as well as other important G protein coupled receptors

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21
Q

Summarize the effects of thyroid hormone on neural activity

A

-> alertness, memory, learning, emotional stability, nerve reflexes

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22
Q

Summarize the effects of thyroid hormone on reproduction & growth

A

-> required for reproductive capabilities
-> enhances effects of growth hormone

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23
Q

Summarize the effects of thyroid hormone on metabolism

A

-> increased heat prod
-> increased lipolysis, glycogenolysis (breaking down glycogen into glucose)
-> increased glucose absorption from GIT
-> increased mitochondrial size, no and enzymes
-> increased basal metabolic rate (rate of O2 consumption/ energy expenditure at rest)

24
Q

Summarize the effects of thyroid hormone on oxygen delivery

A
  • increases heart rate, stroke volume, force of contractions, resting resp rate
  • increased RBC mass
  • up regulates B1 receptors
25
What are beta adrenergic receptors ?
proteins that respond to adrenaline/noradrenaline -> type of G protein coupled receptor
26
Explain step by step how Iodine deficiency can cause hypothyroidism
1. T3 and T4 levels drop due to iodine deficiency (as iodine is combined with tyrosine to create these hormones) which the hypothalamus senses. 2. The anterior pituitary gland releases TRH and TSH in response 3. TSH stimulates the thyroid gland, and since iodine is lacking, the gland cannot response effectively and TSH levels remain high 4. Thyroid enlarges (goitre) leading to thyroid gland hypertrophy 5. persistent low hormone levels = hypothyroidism
27
Explain step by step how grave's disease causes hyperthyroidism
1. Body makes autoantibodies called TSI (thyroid-stimulating immunoglobins) 2. antibodies mimic TSH and bind to TSH receptors on the thyroid gland 3. thyroid is constantly stimulated to produce more T3 and T4 4. Hyperthyroidism- too much thyroid hormones 5. Anterior Pituitary attempts to reduce TSH production to reduce T3 and T4, but since the thyroid is stimulated by TSI, it keeps overproducing hormones anyway
28
What are various causes of hypothyroidism?
failure of the thyroid gland (primary), hypothalamic (tertiary) /anterior pituitary failure (secondary), lack of dietary iodine
29
What are various causes of hyperthyroidism
long-acting thyroid stimulation (Grave's disease), excess hypothalamic/anterior pituitary secretion, hypersecreting thyroid tumour
30
How can radioactive iodine imaging be used to assess thyroid function
1. doctors inject radioisotope, a radioactive form of iodine, into the bloodstream 2. thyroid gland absorbs radioactive iodine like it does normal iodine 3. a gamma camera detects the radiation and produces an image showing how well different parts of the thyroid are working
31
How can radioactive iodine imaging be used to diagnose thyroid disorders, like hypothyroidism
In a normal thyroid, radioactive iodine is evenly taken up in a uniform bow-tie shape In a dysfunctional thyroid, certain areas may not absorb iodine, and appear as 'cold nodule' (dark spots) cold nodules indicate thyroid damage, tumors or hypothyroidism
32
How can urine tests help assess thyroid function?
if thyroids don't absorb much iodine, it will be exreted in urine, hence if high levels of radioactive iodine are detected in urine, this suggests hypothyroidism
33
what are the 3 categories of causes of hypothyroidism
primary - thyroid gland isn't working (autoimmune diseases like Hashimoto's disease, thyroperoxidase, autoantibodies against thyroglobulin) secondary- anterior pituitary isn't releasing TSH tertiary - hypothalamus isn't releasing TRH
34
What are other causes of hypothyroidism?
dietary iodine deficiency, inherited defects of hormone synthesis, defects in TSH receptor, peripheral resistance to thyroxine
35
What are signs and symptoms of hypothyroidism?
fatigue, muscular sluggishness, slow heart rate, reduced cardiac output, weight gain, constipation, mental sluggishness, cold intolerance, thick tongue, hoarseness, goiter, hypercholesterolemia
36
What are causes of hyperthyroidism
the formation of TSI (thyroid stimulating immunoglobin),
37
What are signs and symptoms of hyperthyroidism?
nervousness, tachycardia/tremor, fatigue associated with muscle atrophy, heat intolerance with sweating, weight loss without loss of appetite, increased/decreased goiter
38
Why is calcium vital to the human body?
It maintains cell membrane structure and ability, binds to troponin which allows a process that enables muscle contraction, it triggers the release of neurotransmitters into the synapse, it regulates ion channels and excitability
39
How do levels of calcium regulate the chance of muscle spasms, cramps and seizures?
when blood calcium is low, neuronal membranes become more permeable to sodium, hence causing progressive depolarisation and as a result, neurons can fire action potentials more easily, which can lead to increased neural excitability
40
Why must plasma calcium concentration be maintained within a narrow range?
so appropriate cellular function and excitability is maintained.
41
What 3 hormones are involved in the regulation of plasma calcium concentration
- parathyroid hormone (PTH) - activated vitamin D/Calcitriol - Calcitonin
42
Where are the parathyroid glands located?
on the posterior/back surface of the bow tie shaped thyroid gland.
43
Explain how receptors responds to low calcium levels in the blood using the hormone PTH
calcium sensing receptors CaSR in the thyroid gland detect how much free calcium is in the blood, and if this falls below the normal range, parathyroid glands release PTH
44
PTH AFFECTS KIDNEY BY
- increase Ca reabsorption - less lost in urine - decrease phosphate reabsorption -> more phosphate lost in urine -activate vit D (calcitriol) by increasing enzyme that converts inactive vit D into calcitriol
45
PTH affects the intestine by?
activating vitamin D (calcitriol) to help the small intestine absorb more calcium from food
46
PTH affects bones by?
stimulating bone resorption, causing osteoclasts to break down bone tissue and releasing stored calcium into the bloodstream
47
What is vitamin D synthesised from?
cholesterol derivative when exposed to sunlight
48
What are levels of vitamin D supplemented by?
dietary intake
49
What must vitamin D be activated by before it can exert its effect on instestines?
calcitriol
50
What is the activation of vitamin D increased by?
PTH
51
What does active vitamin D do?
increase absorption of calcium and phosphate in the intestine and boost bone responsiveness to PTH
52
What is calcitonin?
made by C cells of the thyroid gland
53
What is the function of calcitonin
it lowers blood calcium levels, unlike PTH and vitamin D, but is not essential for day-today calcium control, rather being more helpful in special conditions like pregancy/breastfeeding by providing structural integrity when there is high calcium demand.
54
What are the actions of calcitonin?
It inhibits osteoclasts, so less calcium is released from bone It reduces calcium and phosphate reabsorption in the kidneys its net effect is to reduce blood calcium and phosphate levels
55
How does calcitriol increase Ca absorption in the intestine?
1. Increasing expression of calcium channels on the apical membrane 2. Increasing expression of calbindin which ferries calcium across the cell 3. Increasing expression of Ca-ATPase pumps on the basolateral membrane
56
What is the role of calcitonin in regulating blood calcium levels?
When calcium is low, parathyroid glands are stimulated, releasing PTH which acts to increase plasma calcium by

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