Hypothyroid Disorders Flashcards
State 8 of the effects of hypothyroidism.
Reduced BMR: everything slows down Cold intolerance Deepening voice Weight gain with reduced appetite Depression + tiredness Speech slows down Bradycardia Constipation
What are the main thyroid hormones? Which is more active? Which is secreted more?
T3 + T4
More active= T3
Secreted more= T4
What converts T4 to T3?
Deiodinase enzyme
Describe the mechanism of action of thyroxine.
Thyroxine enters the target cell + is converted to T3 by deiodinase
T3 binds to a thyroid hormone receptor in the nucleus + heterodimerises with a retinoid X receptor
This complex binds to a thyroid response element, causing a change in gene expression
What are the 2 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?
Primary hypothyroidism (AI, Iatrogenic- post-thyroidectomy, post-radioactive iodine)
How often is the tablet taken and what measurement is taken to guide the dose?
Once daily
TSH is measured. 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 hypothyroidism
Thyroxine = LOW TSH = HIGH
What is secondary hypothyroidism? What measurement is used to guide the dose in this case?
A problem with TSH production by the adenohypophysis
No problem with thyroid gland itself.
No TSH production, so thyroxine replacement therapy is monitored by measuring free T4 (fT4) levels + keeping it within the reference range
What is the clinical use of liothyronine sodium?
Treatment of myxoedema coma (very rare complication of hypothyroidism)
Give IV liothyronine sodium because the onset of action is faster than T4
Switch to T4 on improvement
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’s difficult to get the correct dosage
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, Osteoporosis
Metabolic: Increased energy expenditure, Weight loss
Cardiac: Tachycardia, Arrythmia
Beta-adrenergic activity: Tremor, Nervousness
What are the half-lives of T3 and T4?
T3 = 2.5 days T4 = 6 days (useful if forget to take tablet)
What plasma protein is T3 and T4 mainly bound to?
Thyroxine binding globulin (TBG)
