Levothyroxine and liothyronine vs PTU and methimazole
How to tell Levothyroxine and liothyronine and PTU and methimazole apart when a board question makes them look alike.
Levothyroxine/liothyronine add thyroid hormone; PTU/methimazole stop thyroid hormone production.
Side by side
Replacement therapy: levothyroxine is T4 for routine hypothyroidism; liothyronine is T3 with faster onset and shorter duration.
Thionamides for hyperthyroidism: PTU and methimazole block thyroid hormone synthesis; PTU also blocks peripheral T4-to-T3 conversion.
Thyroid hormone replacement restores T4/T3 signaling and is titrated mainly by TSH in primary hypothyroidism.
Thionamides inhibit thyroid peroxidase; PTU additionally decreases peripheral T4-to-T3 conversion.
High TSH with low free T4 and long-term dose adjustment points levothyroxine.
Graves disease, thyroid storm, agranulocytosis warning, or first-trimester pregnancy context points thionamide selection.
The trap
Do not treat a hyperthyroid stem with replacement just because thyroid hormone names appear.
The repair move
First decide replacement versus suppression; then choose T4/T3 or PTU/methimazole.
Try a free question
CoreA patient with primary hypothyroidism is treated with synthetic T4, and the dose is adjusted using TSH after steady state. Which medication best fits routine replacement?
Show answer and explanation
Answer: A. Levothyroxine. Levothyroxine is synthetic T4 and is the standard long-acting replacement for hypothyroidism.
Why the other choices are tempting
- B. Liothyronine is synthetic T3 with faster onset and shorter duration.
- C. Propylthiouracil inhibits thyroid hormone synthesis and peripheral T4-to-T3 conversion.
- D. Methimazole inhibits thyroid hormone synthesis.
- E. Potassium iodide acutely blocks hormone release and organification in selected settings.
Flashcards
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What is the fastest separator for Levothyroxine and liothyronine vs PTU and methimazole?
Levothyroxine/liothyronine add thyroid hormone; PTU/methimazole stop thyroid hormone production.
What clue should make you choose Levothyroxine and liothyronine?
Replacement therapy: levothyroxine is T4 for routine hypothyroidism; liothyronine is T3 with faster onset and shorter duration.
Quick answers
What is the fastest way to tell Levothyroxine and liothyronine from PTU and methimazole?
Levothyroxine/liothyronine add thyroid hormone; PTU/methimazole stop thyroid hormone production.
What is the key clue for Levothyroxine and liothyronine?
Replacement therapy: levothyroxine is T4 for routine hypothyroidism; liothyronine is T3 with faster onset and shorter duration.
What is the key clue for PTU and methimazole?
Thionamides for hyperthyroidism: PTU and methimazole block thyroid hormone synthesis; PTU also blocks peripheral T4-to-T3 conversion.
What is the common trap with Levothyroxine and liothyronine vs PTU and methimazole?
Do not treat a hyperthyroid stem with replacement just because thyroid hormone names appear.