side by side comparison
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.
Comparison
Side-by-side2 compared
Dimension
Levothyroxine
Antithyroid Drugs
Class & mechanism
- Synthetic T4 → hormone replacement
- Inhibit hormone synthesis; PTU blocks T4→T3
Indications
- Hypothyroidism, myxedema
- Hyperthyroidism, Graves, thyroid storm
Route & onset
- PO; effect takes weeks
- PO; effect takes weeks
Key assessment
- Apical HR before dose
- Signs of over-replacement (hyperthyroid)
- ★Sore throat or fever — agranulocytosis
- Signs of hypothyroidism (over-treatment)
Monitoring labs
- TSH — goal normal range
- CBC for agranulocytosis; LFTs (PTU)
- Free T4 & TSH to titrate
Adverse effects
- Palpitations, insomnia, weight loss
- Rash, agranulocytosis, hepatotoxicity
Toxicity & black box
- Overdose → tachycardia, tremor, weight loss
- ★PTU: hepatotoxicity / liver failure (black box)
Contraindications & interactions
- ★Separate from calcium/iron/antacids (↓ absorption)
- Potentiates warfarin
- Methimazole teratogenic — PTU in 1st trimester
Patient teaching
- Take AM on empty stomach; same brand, lifelong
- Do not stop abruptly
- Report sore throat, fever, or bruising
- Do not stop abruptly
Class & mechanism
Levothyroxine
- Synthetic T4 → hormone replacement
Antithyroid Drugs
- Inhibit hormone synthesis; PTU blocks T4→T3
Indications
Levothyroxine
- Hypothyroidism, myxedema
Antithyroid Drugs
- Hyperthyroidism, Graves, thyroid storm
Route & onset
Levothyroxine
- PO; effect takes weeks
Antithyroid Drugs
- PO; effect takes weeks
Key assessment
Levothyroxine
- Apical HR before dose
- Signs of over-replacement (hyperthyroid)
Antithyroid Drugs
- ★Sore throat or fever — agranulocytosis
- Signs of hypothyroidism (over-treatment)
Monitoring labs
Levothyroxine
- TSH — goal normal range
Antithyroid Drugs
- CBC for agranulocytosis; LFTs (PTU)
- Free T4 & TSH to titrate
Adverse effects
Levothyroxine
- Palpitations, insomnia, weight loss
Antithyroid Drugs
- Rash, agranulocytosis, hepatotoxicity
Toxicity & black box
Levothyroxine
- Overdose → tachycardia, tremor, weight loss
Antithyroid Drugs
- ★PTU: hepatotoxicity / liver failure (black box)
Contraindications & interactions
Levothyroxine
- ★Separate from calcium/iron/antacids (↓ absorption)
- Potentiates warfarin
Antithyroid Drugs
- Methimazole teratogenic — PTU in 1st trimester
Patient teaching
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
★ marks the fact that sets a column apart.
Clinical Pearl
Toxicity mirrors the opposite disease — levothyroxine overdose looks hyperthyroid, antithyroid overdose looks hypothyroid.
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