Hypothyroid Disorders Flashcards
State some of the effects of hypothyroidism.
Reduced basal metabolic rate – everything slows down Cold intolerance Deep voice Weight gain Loss of appetite Depression Lethargy Speech slows down Bradycardia Constipation
What are the main thyroid hormones? Which is more active?
T3 and T4
T3 is more active but most of the thyroid hormone released by the thyroid gland is in the T4 form
What converts T4 to T3?
Deiodinase
Describe the mechanism of action of thyroxine.
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
This complex then binds to a thyroid response element, which causes a change in gene expression
What are the two main drugs that are used as thyroxine and T3 replacement?
T4 replacement – Levothyroxine Sodium
T3 replacement – Liothyronine Sodium
What is thyroxine replacement used to treat?
Autoimmune primary hypothyroidism
Iatrogenic primary hypothyroidism
How often is the tablet taken and what measurement is taken toguide the dose?
Once daily
TSH is measured and the aim is to use thyroxine replacement to suppress TSH so that it is within the reference range
Describe the levels of thyroxine and TSH in someone with primary thyroid failure.
Thyroxine = LOW TSH = HIGH
What is secondary hypothyroidism? What measurement is used to guide the dose in this case?
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
What is the clinical use of liothyronine sodium?
Treatment of myxoedema coma (very rare complication of hypothyroidism)
You give IV liothyronine sodium because the onset of action is faster than levothyroxine sodium
Why would you give a patient combined thyroid hormone replacement (T3+T4)?
Some patients don’t feel better with T4 replacement alone though their
TSH may be normal
What is the problem with giving T3 replacement?
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
Describe some adverse effects of thyroid hormone over-replacement.
Skeletal Increased bone turnover Reduced bone mineral density Risk of osteoporosis Metabolic Increased energy expenditure Weight loss Cardiac Tachycardia Risk of dysrhythmia Beta-adrenergic activity Tremor Nervousness
What are the half-lives of T3 and T4?
T3 = 2-5 hours T4 = 6 days
What plasma protein is T3 and T4 mainly bound to?
Thyroxine binding globulin