Thyroid Drug Comparison: Levothyroxine vs Antithyroid Drugs — Opposite Conditions, Opposite Monitoring
These drugs are pharmacologic opposites — one replaces what's missing, the other suppresses what's overproduced. When you confuse them, you misread toxicity signs as therapeutic success and miss a sore throat that signals life-threatening agranulocytosis.
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- Synthetic T4 (hormone replacement)
- Inhibit hormone synthesis
- PTU also blocks T4 to T3
- Hypothyroidism, myxedema
- Hyperthyroidism, Graves', thyroid storm
- PO
- effect takes weeks
- IV for myxedema coma
- PO
- effect takes weeks
- Apical HR before dose
- Palpitations, tremor (overdose)
- ★Sore throat or fever — agranulocytosis
- Cold, lethargy, weight gain (overdose)
- TSH in normal range (6–8 wk)
- CBC for agranulocytosis
- LFTs (PTU)
- Free T4 & T3 to titrate
- Palpitations, insomnia, weight loss
- Rash, agranulocytosis, hepatotoxicity
- Black box: not for weight loss
- ★PTU black box: liver failure
- ★Separate from calcium, iron, antacids
- Potentiates warfarin (↑ INR)
- 1st trimester: PTU, not methimazole
- Take AM on empty stomach
- Same brand, lifelong
- Do not stop abruptly
- Report sore throat, fever, or bruising
- Do not stop abruptly
Levothyroxine
- Synthetic T4 (hormone replacement)
Antithyroid Drugs
- Inhibit hormone synthesis
- PTU also blocks T4 to T3
Levothyroxine
- Hypothyroidism, myxedema
Antithyroid Drugs
- Hyperthyroidism, Graves', thyroid storm
Levothyroxine
- PO
- effect takes weeks
- IV for myxedema coma
Antithyroid Drugs
- PO
- effect takes weeks
Levothyroxine
- Apical HR before dose
- Palpitations, tremor (overdose)
Antithyroid Drugs
- ★Sore throat or fever — agranulocytosis
- Cold, lethargy, weight gain (overdose)
Levothyroxine
- TSH in normal range (6–8 wk)
Antithyroid Drugs
- CBC for agranulocytosis
- LFTs (PTU)
- Free T4 & T3 to titrate
Levothyroxine
- Palpitations, insomnia, weight loss
Antithyroid Drugs
- Rash, agranulocytosis, hepatotoxicity
Levothyroxine
- Black box: not for weight loss
Antithyroid Drugs
- ★PTU black box: liver failure
Levothyroxine
- ★Separate from calcium, iron, antacids
- Potentiates warfarin (↑ INR)
Antithyroid Drugs
- 1st trimester: PTU, not methimazole
Levothyroxine
- Take AM on empty stomach
- Same brand, lifelong
- Do not stop abruptly
Antithyroid Drugs
- Report sore throat, fever, or bruising
- Do not stop abruptly
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Toxicity mirrors the opposite disease — levothyroxine overdose looks hyperthyroid, antithyroid overdose looks hypothyroid.
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