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The Patient > Thyroid > Flashcards

Thyroid Flashcards

1
Q

What is the structure of the thyroid gland?

A

follicular. cells create follicles that are filled with thyroglobulin. cells secrete t3 and t4

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2
Q

What do parafollicular cells secrete?

A

calcitonin

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3
Q

What does thyroglobulin contain?

A

iodide - approx enough for 90 day supply

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4
Q

What is the HPA pathway for thyroid regulation?

A

Hypothalamus releases thyrotropin releasing hormone (TRH)
A. pituitary (pituitary thyrotropes) releases Thyroid stimulating hormone (TSH)
thyroid gland produces more T3 and T4

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5
Q

What are the negative feedback loops in thyroid regulation?

A

sufficient T3 and T4 inhibit production of TRH and TSH (only T3 acts on hypothalamus)

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6
Q

What is the difference between T3 and T4?

A

T4 is far more abdundant, but T3 is more active - T4 is a storage facility and is readily converted to T3 when required

T4 has a much longer half life

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7
Q

What effect does TSH have directly on the thyroid gland?

A

enlargement of thyroid to increase activity

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8
Q

What types of hormones at T3 and T4?

A

thyronine hormones

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9
Q

TSH action on thyroid gland receptors?

A

TSH receptors on follicular cells are GPCRs

  • activation of adenylyl cyclase which produces cAMP
  • cAMP activates all functional aspects of thyroid cell -
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10
Q

What are the functional aspects of the thyroid cells?

A

thyroglobulin synthesis, iodide pumping, iodination by peroxidase, endocytosis, proteolysis and hormone release

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11
Q

What are the steps of iodide transport in follicular cells?

A

Na+ and I- symporter actively transports iodide across basolateral membrane from blood into cells
(Na+ then pumped out by Na+/K+ pump)

Iodide then transported from cells into follicle lumen via pendrin transporter (exchanges chloride for iodide)

iodide then oxidised to Iodine

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12
Q

How is thyroglobulin made?

A

synethsised in endoplasmic reticulum of follicular cells, then secreted (exodytosis) into follicle

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13
Q

Why is thyroglubulin made up of?

A

A Tg backbone and tyrosine molecules

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14
Q

What makes up the thyroid peroxidase complex (TPO)?

A

Tyrosine peroxidase enzyme and iodine

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15
Q

What does the TPO complex do?

A

covalently bind iodine to the tyrosine residues

produces mono- or di-iodotyrosine

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16
Q

How are T3 and T4 produced?

A

conjugating two molecules of MIT / DIT

2 x DIT makes `t4, DIT then MIT makes T3

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17
Q

THe order of steps of thyronines production?

A
  1. iodination of tyrosine
  2. conjugation
  3. endocytosis into follicular cell
  4. proteolysis - cleavage from thyroglobulin
  5. released into bloodstream
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18
Q

Another name for T4?

A

thyroxine

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19
Q

What is reverse T3?

A

inactive. reverse tri-iododothyronine

less than 1% of all thyronines

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20
Q

What drugs are used to treat hypothyroidism?

A

levothyroxine (T4) - tablets and oral solution

liothyronine (T3) - tablets and slow iv injection

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21
Q

What anti-thyroid drugs are available?

A

carbimazole (brand name neo-mercazole) - tablets
propylthyrouracil - tablets

block synthesis of thyroid hormones

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22
Q

How is T4 transported in the blood and why?

A

insoluble in serum, so travels bound to proteins

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23
Q

How can liver disease lead to thyroid related problems?

A

transport proteins are synthesised in liver, so not enough can lead to loss of effective T4 transport to peripheral tissues

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24
Q

How much thyroid hormone does Thyroxine binding globulin (TBG) bind, and what is the half life?

A

70-75% of plasma T4, as well as T3

T1/2 = 5 days

large circulating T4 resevoir, and prevents loss of T4 in urine

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25
Q

How much thyroid hormone does Transthyretin bind, and what is the half life?

A

20% of plasma T4, no T3

half life 2-3 days

important for CNS delivery

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26
Q

How much thyroid hormone does albumin bind?

A

5-10% of plasma T4, and 30% of T3

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27
Q

What are the benefits of protein bound thyroid hormones?

A
  • transport
  • prolongs availability of T4 to tissues
  • buffer effects of altered T4 secretion from thyroid
  • can control delivery of T4 to tissues (rate etc)
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28
Q

How do the thyroid hormones enter cells?

A

must be unbound

through specific transporters (e.g. MCT8, MCT10, OATP1c1)

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29
Q

How is T4 converted to T3 to become more active?

A

intracellular iodothyronine deiodinases

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30
Q

What are iodothyronine deiodinases made of?

A

seleno-cysteine containing enzymes, selenium accepts iodine

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31
Q

How many iodothyronine deiodinases are there?

A

3 - DIO1, DIO2 and DIO3

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32
Q

What is DIO1?

A

found mostly in liver, kidney and muscle (as well as thyroid)
produces most of the circulating T3

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33
Q

What is DIO2?

A

found mostly in CNS areas and pituitary thyrotropes

controls intracellular T3, important for feedback regulation

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34
Q

What is DIO3?

A

produces inactive (reverse) T3

prevents thyroid hormone access to certain tissues

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35
Q

How do the thyroid hormones exert an effect once inside a cell?

A
  • bind to thyroid hormone receptors (A or B) found in the nucleus, heterodimered with retinoid X receptor (RXR)
  • binds to thyroid responsive element (DNA sequences)
  • transcription factor, or can inhibit

thyroid receptors have 15x higher affiniy for T3 than T4

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36
Q

What effects can T3 exert on the pituitary?

A

gene that codes for growth hormone is T3 responsive, as is the one that codes for decreased prolactin and less TSH (feedback loop)

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37
Q

What is the importance of maintaining calcium levels in tthe body?

A
  • growth/maintenance of skeletal system
  • neurotransmitter release
  • muscle contraction
  • hormone secretion
  • blood clotting
  • intracellular signalling
  • apoptosis
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38
Q

What are examples of calcium rich foods?

A

dairy, broccoli, almonds, flax, sesame, kale etc

avoid alcohol and smoking as these can inhibit absorption

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39
Q

What regulates calcium levels?

A

Calcitonin and parathyroid hormone

vitamin D and extracellular calcium concentration are also important factors

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40
Q

Which parts of the body play a large role in the calcium in the body?

A

Bone, gut and kidneys

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41
Q

What is the role of the kidneys in the body’s calcium levels?

A

filtration and reabsorption

the kidneys excrete around 200mg calcium every day

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42
Q

What is the role of the gut in the bodys calcium levels?

A

adsorption of calcium into blood, or secretion.

gut is responsible for uptake of calcium from external sources

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43
Q

What happens when calcium regulation is abnormal?

A

disease

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44
Q

What do osteoblasts do?

A

BUILD BONE

use plasma Ca and other minerals

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45
Q

What do osteoblasts do?

A

BUILD BONE

use plasma Ca and other minerals

46
Q

What do osteoclasts do?

A

break down bone (crush) by demineralising

critical for maintenance and repair of bones

47
Q

What diseases are related to defective calcium homeostasis?

A
  • primary/secondary (renal failure) hyperparathyroidism
  • osteoporosis
  • rickets
  • calcium stones
  • receptor mutations (PTH receptor, calcium receptors etc)
48
Q

what type of hormone is calcitonin?

A

protein (32 amino acids)

49
Q

Where is calcitonin produced?

A

parafollicular cells in the thyroid

found between the follicles

50
Q

What is the half life of calcitonin?

A

5 minutes

51
Q

What is the function of calcitonin?

A

Reduce blood calcium levels

- opposes effects of parathyroid hormone

52
Q

Where is the parathyroid?

A

glands attached to the back of the thyroid - 4 of them

53
Q

What are the two types of cells in the parathyroid glands?

A 
chief cells (few in number)
oxyphil cells (many, unknown function)
54
Q

What do chief cells in the parathyroid do?

A

produce parathyroid hormone

55
Q

What is the structure of parathyroid hormone? half life?

A

helical protein, 84 amino acids

half life 20 mins

56
Q

What is the function of parathyroid hormone?

A

increase blood calcium levels

57
Q

How are low blood calcium levels detected?

A

the calcium sensing receptors (CaSR) - a GPCR

58
Q

What effect does parathyroid hormone have on the body?

A
  • stimulate osteoclasts to release more calcium from bones (indirectly), resorption (PTH + Vit D)
  • increase renal calcium reabsorption (PTH + Vit D)
    • PTH also stimulates vit D activation in kidneys
  • increase production of vitamin D, which helps improve intestinal absorption of calcium
59
Q

What is the parathyroid hormone feedback loop?

A

INcrease in blood calcium reduces secretion of PTH

60
Q

What is important to note about impact of PTH on bones?

A

Intermittent PTH causes bone formation

61
Q

What effect does calcitonin have on the body?

A

Reduce osteoclast acitivity (reduced bone resorption), allows rapid bone deposition by osteoclasts

62
Q

What is the calcitonin feedback loop?

A

decreased blood calcium decreases calcitonin secretion, and reduces bone formation

63
Q

Connection between calcitonin levels and disease?

A

Effects of calcitonin are minor, and high or low levels do not cause disease

64
Q

Which cells are responsible for the action of PTH on bone resorption?

A

A substance released from osteoblasts in response to PTH causes osteoclasts to release calcium from bone

65
Q

In which gland does primary thyroid disease occur?

A

Thyroid

66
Q

In which gland does secondary thyroid disease occur?

A

Pituitary

67
Q

In which gland does tertiary thyroid disease occur?

A

Hypothalamus

68
Q

What are the causes of primary hypothyroidism?

A
  • Autoimmune (Hashimoto’s thyroiditis)
  • previous treatment for hyperthyroidism
  • iodine imbalance (uncommon in UK)
  • congenital hypothyroidism
69
Q

Symptoms of hypothyroidism?

A

lethargy, cold, memory loss, weight gain, gruff voice, depression, constipation, hair loss, dry skin

70
Q

What markers can be tested to look for primary hypothyroidism?

A

TSH and unbound T4 - also thyroid peroxidase antibody but rarely measured outside secondary care

primary hypothyroidism has high TSH and low T4

71
Q

Treatment options for someone symptomatic with high (5-10) TSH and normal T4?

A

trial 3-6 months thyroxine

if symptoms resolved: lifelong treatment
if symptoms not resolved, consider alternate diagnoses (thyroid peroxidase antibody check)

72
Q

Treatment options for someone asymptomatic with high (5-10) TSH and normal T4?

A

check thyroid peroxidase antibody

positive: check thyroid hormones annually
negative: check thyroid hormones every 3 years

73
Q

Treatment options for someone with high (5-10) TSH and low T4?

A

treat with lifelong thyroxine

74
Q

Treatment options for someone with v high (10+) TSH, with or without low T4

A

treat with lifelong thyroxine

75
Q

Half life of T4?

A

7 days

76
Q

Initial dose of thyroxine treatment in adults under 50?

A

initially 50-100 mcg thyroxine daily, adjusted 25–50mcg every 3–4 weeks according to response

77
Q

Initial dose of thyroxine treatment in adults over 50, or in heart disease?

A

initially 25mcg once daily, adjusted 25mcg every 3–4 weeks according to response

78
Q

Initial doses of thyorxine treatment for congenital hypothyroidism?

A

initially 10-15 mcg/kg for neonates (max 50 mcg), adjusted 5 mcg/kg

79
Q

How soon after starting treatment should TSH be re-measured?

A

8-12 weeks

80
Q

What is the usual maintenance dose for thyroxine treatment for adults?

A

100-200mcg

81
Q

What is the usual maintenance dose for thyroxine treatment for children?

A

50-200mcg

age dependent

82
Q

What should be monitored during ongoing thyroxine treatment?

A

TSH - should be in lower half of reference range
lack of symptoms

also monitor for angina

83
Q

When is liothyronine used in combination with thyroxine?

A

rarely - only by endocrinologist. meta-analysis shows no obvious benefit

84
Q

What counselling points are important for patients taking thyroxine?

A

single daily dose, taken for life
don’t take at the same time as calcium or iron preps
there are three strengths of tablet - do not confuse
importance of regular monitoring
medical exemption

85
Q

What causes most cases of hyperthyroidism?

A

Grave’s disease (autoimmune)

86
Q

How does grave’s disease cause hyperthyroidism?

A

The body produces antibodies to the TSH receptor, which stimulate the gland

87
Q

What is the prevalence of Grave’s disease?

A

2% of women, 0.2% of men

causes 3 of 4 hyperthyroidism cases

88
Q

What are symptoms of hyperthyroidism?

A
  • anxious, tremor
  • tachycardia, palpitations
  • weight loss
  • goitre
  • prefers cold weather
  • warm clammy skin
89
Q

What tests are done to diagnose hyperthyroidism?

A

TSH and unbound T4

low TSH and high T4 indicate hyperthyroidism

90
Q

What are the treatment options for hyperthyroidism?

A

drug treatment, radioactive iodine, surgery

none are ideal - patient choice where possible

91
Q

What patient groups is drug therapy for hyperthyroidism preferred for?

A

children, pregnancy/breastfeeding, uncomplicated disease in young adults, acute phase before surgery

92
Q

How do thionamide drugs work?

A

e.g. carbimazole , propythiouracil

inhibit coupling of iodotyrosines etc

93
Q

What is the normal regimen for carbimazole?

A

15-40mg daily (severity depending, can be higher)
maintain 4-8 weeks until TFTs normal

maintenance for 12-18 months: reduce by 20-30% each month until 5-15mg daily

94
Q

What is the blocking replacement regimen for carbimaole

A

40-60mg for 4 weeks
then add thyroxine 50-100mcg (makes patient hypothyroid temporarily)
continue for -18 months ish
thyroid function returns to normal when stop medication

95
Q

What is important with carbimazole in pregnancy?

A

Blocking replacement regimen is not suitable - high doses can be dangerous for fetus
- crosses placenta

96
Q

What is the equivalent of carbimazole and propythiouracil?

A

1mg carbimazole = 10mg propythiouracil

97
Q

What are the inital and maintenance doses and regimens of propythiouracil ?

A

200-400mg daily initially, divided doses

maintenance 50mg three times a day

98
Q

What is important with propythiouracil in pregnancy?

A

potentially slightly safer but unknown

99
Q

When is propythiouracil used?

A

carbimazole not tolerated/agranulocytosis

100
Q

What is drug induced agranulocytosis?

A

carbimazole and propythiouracil can both cause bone marrow suppression

reduced white cell count leads to infection

occurs in 0.3-0.5% of cases, not dose dependent

101
Q

What counselling points are important for hyperthyroid patients?

A
  • frequency of dosing difference
  • tapering to maintenance dose
  • signs of agranulocytosis (sore throat, bruising, mouth ulcers, any infection sign)
  • signs of hepatic dysfunction
  • need for regular tests
  • management of relapse
102
Q

When is radioactive iodine treatment indicated for hyperthyroidism?

A

failure of treatment, relapse or toxic nodular goitre

103
Q

When is surgery indicated for hyperthyroidism?

A

oesophageal obstruction, intolerance to drugs, young age

104
Q

Why should radioiodine and surgery not be used in the acute stage of disease?

A

can cause thyrotoxic crisis (thyroid storm) - excessive release of thyroid hormones when damaging the gland, can be fatal

105
Q

What adjuvant therapies are used in hyperthyroidism treatment?

A

beta-blockers to reduce CNS symptoms
multiple daily dosing due to increased metabolism

usually only needed in initial stages when still symptomatic

106
Q

What drugs can cause drug-induced thyroid disease?

A

iodine, amiodarone, lithium

107
Q

How can iodine cause drug-induced thyroid disease?

A

overdose from radiographic media
acute phase- inhibit release
prolonged- suppression of production

can cause thyrotoxicosis if problem with autoregulation

iodine deficiency can cause hypothyroidism

108
Q

How does amiodarone cause hypothyroidism?

A

can cause inhibition of release of T3 and 4

usually continue amiodarone and use T4 replacement therapy if needed

109
Q

How does amiodarone cause hyperthyroidism?

A

contains iodine
MILD: blocks conversion of T4 to T3, so increase of TSH and T4
usually resolves itself in a few months after starting treatment

SEVERE: increased T4 production (due to iodine) - direct thyroiditis.
should withdraw therapy if possible, or use carbimazole

110
Q

How does lithium cause hypothyroidism?

A

inhibits iodine uptake and prevetns release of T3 and 4

can be transient and sub-clinical, so monitor TSH and start therapy if it becomes clinical

111
Q

How can lithium cause hyperthyroidism?

A

rare, paradoxical effect

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