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Y2 LCRS1 Endocrinology > Hypothyroid Disorders > Flashcards

Hypothyroid Disorders Flashcards

1
Q

Draw the HPT axis.

A
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2
Q

What is primary hypothyroidism? What is it also known as?

A

Myxoedema

Autoimmune damage to thyroid –> thyroxine levels decline–> TSH levels rise

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3
Q

State some of the effects of primary hypothyroidism.

A
  • Deepening voice
  • Depression and tiredness
  • Cold intolerance
  • Weight gain with reduced appetite
  • Constipation
  • Bradycardia
  • Eventual myxoedema coma
  • Reduced basal metabolic rate – everything slows down
  • Speech slows down
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4
Q

Describe the formation and release of thyroxine by thyroid follicles.

A
  • Pituitary gland makes TSH, which turns on the trapping of iodide. Iodide goes into the cell and is converted to iodine.
  • This iodine binds to an amino acid called tyrosine which is then converted to thyroxine.
  • The pink in the middle is the colloid which stores thyroxine. (The store would last a month without making any new thyroxine.)
  • When you need some thyroxine, TSH turns on an enzyme which releases the protein so that thyroxine is released into circulation to warm you up. (BMR)
  • T3 is the active form. T4 is converted to T3 by removing an iodine atom and this happens all around the body. There is negative feedback.
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5
Q

What are the main thyroid hormones? Which is more active?

A

T3 - tri-iodothronine

T4 - thyroxine/tetraiodothyronine

  • T3 is more active but most of the thyroid hormone released by the thyroid gland is in the T4 form
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6
Q

What converts T4 to T3?

A

Deiodinase (removes an iodine atom from T4)

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7
Q

What are the two sources of T3 in the circulation?

A

T3:

  • 80% from deiodination of T4
  • 20% from direct thyrdoidal secretion

(T3 provides almost all thyroid hormone activity in target cells)

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8
Q

Describe the mechanism of action of thyroxine.

A

Thyroxine enters the target cell and is converted to T3 by deiodinase

T3 then binds to a thyroid hormone receptor in the nucleus and then heterodimerises with a retinoid X receptor (RXR)

This complex then binds to a thyroid response (TRE) element, which causes a change in gene expression

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9
Q

What are the two main drugs that are used as thyroxine and T3 replacement?

A

T4 replacement – Levothyroxine Sodium

T3 replacement – Liothyronine Sodium (less commonly used)

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10
Q

What is thyroxine replacement used to treat?

A
  1. Primary hypothyroidism - e.g. autoimmune , iatrogenic - post-thyroidectomy, post-radioactive iodine.
  2. Secondary hypothyroidism - e.g. pituitary tumour, post pituitary surgery or radiotherapy.
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11
Q

How is levothyroxine administered?

A

Orally

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12
Q

Describe the levels of thyroxine and TSH in someone with primary thyroid failure.

A

Thyroxine = LOW

TSH = HIGH

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13
Q

What is secondary hypothyroidism? What measurement is used to guide the dose in this case?

A

This is a problem with TSH production by the adenohypophysis

There is no problem with the thyroid gland itself

As there is no TSH production, thyroxine replacement therapy is monitored by measuring free T4 (fT4) levels and keeping it within the reference range

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14
Q

How do you monitor how much levothyroixne to administer?

A
  • Primary hypothryoidism (e.g. autoimmune) - TSH used as guidance - aim to suppress TSH into reference range.
  • Secondary hypothyroidism - can’t use TSH since it is liw in anterior pituitary failure; so aim for fT4 (free thyroxine) middle of reference range.
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15
Q

What is the clinical use of liothyronine sodium? How is it adminstered and why?

A

Treatment of myxoedema coma (very rare complication of hypothyroidism)

You give IV liothyronine sodium because the onset of action is faster than levothyroxine sodium (T4) then oral when possible.

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16
Q

Why would you give a patient combined thyroid hormone replacement (T3+T4)?

A

Some patients don’t feel better with T4 replacement alone though their TSH may be normal

17
Q

What is the problem with giving T3 replacement?

A

T3 is very potent so it is difficult to get the dose right

Too high a dose can lead to patients complaining of thyrotoxicosis type symptoms:

  • palpitations,
  • tremor,
  • anxiety

(Often combination treatment suppresses TSH)

18
Q

What are the half-lives of T3 and T4?

A

T3 = 2-5 hours

T4 = 6 days

19
Q

What plasma protein is T3 and T4 mainly bound to? What percentage?

A

Thyroxine binding globulin (DO NOT CONFUSE WITH THYROGLOBULIN)

99.7% of circulating T3 and T4

NB: only free hormone is available to tissues

20
Q

What can cause an increase in the production of plasma proteins?

A

Pregnancy

Prolonged treatment with oestrogen and phenothiazines

21
Q

What can cause a decrease in the amounts of the plasma proteins?

A

Liver failure (most plasma proteins are produced by the liver)

Severe malnourishment

Certain co-administered drugs compete for protein binding sites e.g. phenytoin, salicylates.

22
Q

Select True or False for each of the following statements:

a) The thyroid gland is situated in the neck
b) Thyroxine is synthesised from the amino acid tyrosine
c) T3 has a longer half life than T4
d) Hypothyroidism can be due to an autoimmune cause
e) Hyperthyroidism can be due to an autoimmune cause

A
  • True
  • True
  • False
  • True
  • True
23
Q

Select True or False for each of the following statements:

  • a) Patients with severe hypothyroidism usually have a low plasma TSH level
  • b)Tc99m (technetium pertechnetate) scanning shows decreased overall uptake in Graves’ disease
  • c) Tc99m scanning shows decreased overall uptake in Plummer’s disease
  • d)Tc99m scanning shows decreased overall uptake in viral (De Quervain’s) thyroiditis
  • e) Cretinism occurs when the thyroid gland fails during puberty
A
  • False
  • False
  • False
  • True
  • False
24
Q

Select True or False for each of the following statements:

    • Alpha-adrenoceptor blocking drugs are useful in patients with hyperthyroidism
    • Beta-adrenoceptor blocking drugs are useful in patients with hyperthyroidism
    • Alpha-adrenoceptor blocking drugs are useful in patients with hypothyroidism
    • Beta-adrenoceptor blocking drugs are useful in patients with hypothyroidism
    • TRH administration may be useful in patients with primary hypothyroidism
A
  • False
  • True
  • False
  • False
  • False
25
Q

Select True or False for each of the following statements:

    • Hyperthyroidism is associated with weight gain
    • Hyperthyroidism is associated with an increased appetite
    • Surgery for hyperthyroidism or thyroid cancer can cause hyponatraemia
    • Surgery for hyperthyroidism or thyroid cancer can cause hypocalcaemia
    • Surgery for hyperthyroidism or thyroid cancer can cause hypokalaemia
A

False
True
False
True
False

26
Q

Select True or False for each of the following statements:

    • Lid lag is a feature of hyperthyroidism
    • An increase in basic metabolic rate is a consequence of excessive dosing with thyroxine
    • An overdose of thyroxine to a normal individual will cause lid lag
    • T4 has a short plasma half-life of 2-3 minutes
    • T4 is the major circulating thyroid hormone
A
  • True
  • True
  • True
  • False
  • True
27
Q

Select True or False for each of the following statements:

    • T4 has a greater binding affinity for the thyroid hormone receptor than T3
    • Thyroxine increases cardiac output and heart rate
    • Carbimazole is a drug which enhances secretion of thyroid hormones
    • The clinical effects of the thiourylene drugs are immediate
    • A large excess of iodine will cause short term hypothyroidism
A
  • False
  • True
  • False
  • False
  • True
28
Q

What is pseudohypothyroidism?

A

Rare condition
Can’t see the thyroid but it is there

29
Q

What does high calcium and 0 PTH indicate?

A

Malignancy of hyperthyroid
Cancer is taking calcium out of bone
No PTH since Ca is high

Y2 LCRS1 Endocrinology (18 decks)

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