4. Hypothyroid Disorders Flashcards
Clinical name given to primary hypothyroidism
Myxoedema:
- Results from autoimmune damage
- Causes reduced T4
- Presents as an increased level of TSH
Action of TSH in the thyroid gland
TSH ‘switches on’ follicular cells in the gland, causing release of thyroxine into your bloodstream
Having been released histology shows white bits of colloid where the thyroxine was once stored
Effects of primary hypothryoidism
- Thickening of the tongue
- Slowed speech
- Deepening of the voice
- Fall in BMR
- Bradycardia
- Depression
- Cold intolerence
- Weight gain and reduced appetite
- Myxoedemal coma
T4 conversion to T3
(Healthy adult thyroid gland secretes both T4 and T3)
T4 is a prohormone converted to T3 by deiodinase
80% of circulating T3 is produced by deiodination of T4, it provides almost all thyroid hormone activity
T4
Tetraiodothyronine, thyroxine
Thyroid hormone mechanism of action
Having been deiodinated T3 moves to the nucleus of the target cell and binds to the thyroid hormone receptor
It then heterodimerises with a retinoid X receptor- complex binds to a thyroid response element that causes a change in gene expression.
Thyroid Hormone replacement therapy
The usual thryoxine replacement is LEVOTHYROXINE SODIUM
Rarely, a T3 replecement is used- LIOTHYRONINE SODIUM (three
Clinical uses and guidance for levothyroxine sodium
Clinical usage:
- Autoimmune primary hypothyroidism
- Iatrogenic primary hypothyroidism (post thyroidectomy or post-radioactive iodine)
Guidance:
Tablet tends to be taken once a day orally. TSH level is used as a guidance for dosage- want to supress TSH into the reference range. Treatment is also used for SECONDARY hypothyroidism
Secondary Hypothyroidism
- This is a problem with the anterior pituitary
- The thyroid gland is fine (do not produce any TSH)
- Guidance for replacement therapy therfore doesnt exist
- So adjust the range to achieve a fT4 in the middle of the reference range
Clinical use of liothyronine (T3)
- MYXOEDEMA COMA- a rare complication of hypothyroidism
- Need to give I.V Liothyronine (T3) because its onset of action is faster than T4
Combined thyroid hormone replacement
Some patients will not feel better without a combined treatment. However T3 is very potent so dosage can be difficult to get right
Potential complications of combined thyroid hormone replacement
Symptoms of thyrotoxicosis:
- palpitations
- tremor
- anxiety
Adverse effects of thyroid hormone over relacement
Usually associated witha low/ supressed TSH
THYROTOXICOSIS
- Skeletal
- Increased bone turnover
- Reduction in bone mineral density
- Risk of osteoperosis
- Cardiac
- Tachycardia
- Risk of dysrrhythmia
- Metabolism
- Increased energy expenditure
- Weight loss
- Increased B adrenergic sensitivity
T4 half life
6 days
T3 half life
2-5 hours
