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.

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Comparison

Side-by-side2 compared
Dimension
Levothyroxine
Antithyroid Drugs
Class & mechanism
  • Synthetic T4 then 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 then 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 then 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 then 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.

Component Topics

Antithyroid DrugsLevothyroxine / Synthroid

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