Thyroid Drugs Flashcards
(37 cards)
What are the 2 thyroid hormones and what is their relationship?
L-thyroxine (T4) and Liothyronine (T3). T4 is the precursor to T3 which is the active hormone.
Describe the steps in thyroid hormone synthesis.
Peroxidase (enzyme) –> oxidation of dietary iodine –> iodination of tyrosine –> thyroid hormone
How and where is thyroid hormone stored?
Stored as thyroglobulin in the thyroid
Describe the hypothalamic-pituitary-thyroid axis.
Hypot releases TRH –> AP releases TSH –> thyroid releases T4 –> T4 deiodinated to T3 in periphery.
How is the majority of T3 and T4 present in the blood?
Bound to plasma protein (inactive) –> thyroxine binding globulin (TBG)
What increases and decreases TBG?
Inc: pregnancy and oral contraceptives
Dec: anabolic steroids (T inc metabolic requirements which increases need for free T3)
Why are most thyroid meds given PO?
T4 and T3 are lipophilic meaning they are well absorbed in the gut.
What is the basic mechanism of Graves disease?
Autoimmune antibodies that agonize TSH receptors
Describe the pharmacological effects of T3 and T4.
Activating hormones –> stimulate protein synthesis, inc metabolic rate and O2 consumption, inc sensitivity to catecholamines
State and describe the drug most used to treat hypothyroid.
Levothyroxine (T4, aka Synthroid) –> slow on and slow off. Max effect of one dose reached in 10 days.
What is the name of T3 and why is it not used in the treatment of hypothyroid?
Liothyronine –> 5x as potent as T4, max effect in 24h. Its potency makes it too unpredictable.
What lab value indicates hypothyroidism (cretinism)?
High TSH, low T3 and T4
What is the dosing regimen for hypothyroidism?
50-100 mcg synthroid qd titrated up to normal TSH
In dosing synthroid what considerations are given to pregnant females and to cardiac patients?
pregnant: need higher dose (TBG increases)
cardiac: need lower dose (don’t overstimulate heart)
Why is synthroid taken on an empty stomach?
Synthroid sticks to food, dec absorption
When would T3 be administered as treatment for hypothyroid?
Emergency –> myxedema coma
What are potential adverse effects of Synthroid?
inc metabolic rate (pseudohyperthyroidism)
atrial fibrillation
T/F: Brand Synthroid is preferred over generic.
True
What happens if synthroid is dosed too high and thyroid hormone levels rise too much?
D/C drug for 7 days then recommence at lower dose
T/F: Hyperthyroid disease is much easier to treat than hypothyroid disease.
False –> hypothyroid easy to manage by titrating Synthroid. Hyperthyroid often involves surgical removal of the thyroid gland.
What is the mechanism of action of methimazole?
Blocks formation of thyroid hormones by inhibiting oxidation of dietary iodine.
T/F: Methimazole reduces conversion of T4 to T3 in peripheral tissues.
False –> has no effect on T4 - T3 conversion
Why is methimazole not typically used in the first trimester of pregnancy?
It easily crosses the blood-placenta barrier.
What is the effect of plasma albumin concentration on the action of methimazole?
No effect –> methimazole is not plasma protein bound.
