Thyroid medications Flashcards
Define:
Euthyroid, hypothyroidism, hyperthyroidism, thyroiditis, thyrotoxicosis
Euthyroid - normal levels of thyroid function
Hyperthyroidism- too much thyroid hormone function
Hypothyroidism - too little thyroid hormone function
Thyroiditis - inflammation of the thyroid gland
Thyrotoxicosis - state caused by too much thyroid hormone in the body
Function of the thyroid gland
secretes hormones that regulate human growth and development by modulating a number of metabolic and physiological processes
Levothyroxine (Thyroxine, T4) mechanism of action
Levothyroxine is a synthetic form of the natural T4 hormone, thyroxine.
Although levothyroxine is a synthetic form of T4, it serves as a source of T3 within the body by deiodination (approximately 80% of T3 is derived locally from T4 by conversion in target tissues).
Levothyroxine is generally considered the treatment of choice for hypothyroidism because it provides both T4 and T3.
When should levothyroxine be taken? Who has to take a higher dose?
Should be taken on an empty stomach
Infants need higher dose because metabolise T4 faster (slows with age)
Levothyroxine drug interactions
Aluminum-containing antacids, calcium carbonate, and proton pump inhibitors can also interfere with the absorption of thyroid hormones.
Cholestyramine can bind thyroid hormones in the intestine and block absorption.
Thyroid hormones stimulate the catabolism of vitamin K-dependent clotting factors and so can influence the action of various anticoagulants.
Black box warning for thyroid medications
None should be used for weight loss
Liothyronine (Triiodothyronine, T3) mechanism of action
Liothyronine is a synthetic form of the natural T3 hormone.
Because there is no need to metabolize liothyronine to an active drug it has a more rapid onset of action than levothyroxine.
As the active drug, liothyronine is approximately 4 times as potent as levothyroxine.
What is different about liothyronine concentrations
The half-life of liothyronine is shorter than that of levothyroxine, so liothyronine supplements are generally taken multiple times a day compared to once daily for levothyroxine.
Rapid onset together with shorter half-life means there can be wider concentration swings with liothyronine leading to more fluctuation in clinical response.
Why does liothyronine have limited use
Concentration swings tend to offset any advantages seen with liothyronine compared to levothyroxine and therefore limit its use.
Liothyronine is also more expensive than levothyroxine.
What is liotrix? Is it used?
A 4/1 mixture of T4 and T3, respectively meant to more closely mimic thyroid output)
There is no therapeutic benefit reported for liotrex compared to levothyroxine and there is an increased risk for elevated T3 concentrations when using liotrix.
Two main approaches for treating hyperthyroidism:
1) Reducing the amount of thyroid tissue capable of producing thyroid hormones
2) Reducing the amount of thyroid hormones produced from pre-existing thyroid tissue.
Why is thyroid ablation considered a good option for hyperthyroidism?
it is generally considered easier and safer to treat hypothyroidism by hormone replacement therapy that in is to treat hyperthyroidism by non-ablative approaches
What is the coarse of action for abblation by radiotherapy and for thyroid cancer
Radioactive iodide ( 131I) is a β particle emitter with a half-life of 8 days, selectively toxic for thyroid cells
What is thyroid peroxidase (TPO)
TPO is a heme-containing peroxidase that catalyzes the iodination of tyrosine residues in thyroglobulin (Tg) and the subsequent phenolic coupling of iodinated tyrosine residues to give Tg-bound precursors of thyroxine (T4) and tri-iodothyronine (T3)
What do thioamides do?
Thioamide drugs inhibit thyroid peroxidase (TPO) blocking the iodination of tyrosine residues in thyroglobulin and the phenolic coupling of iodinated tyrosine residues to form precursors of T4 and T3.
