☀️ Methimazole/PTU Antithyroid Drug Therapy Simulator

Untreated Graves' Hyperthyroidism

Thyroid peroxidase drives excess hormone output before treatment begins.

• High: Free T4 (above normal range)
• Low: TSH (suppressed by feedback)
• 100%: TPO activity (fully active enzyme)
• Present: Symptoms (tachycardia, weight loss)

Why hormone output is high

Autoantibodies stimulate the TSH receptor, driving unchecked hormone synthesis.

Role of thyroid peroxidase

TPO iodinates thyroglobulin, the rate-limiting step of hormone production.

Thionamide Binds Thyroid Peroxidase

Methimazole or PTU molecules begin occupying the enzyme active site.

• Thionamide: Drug class (methimazole or PTU)
• TPO enzyme: Target (iodination step)
• Days: Onset (before effect seen)
• Hours: Half-life (varies by drug)

Mechanism of blockade

Drug molecules occupy TPO, preventing iodine attachment to thyroglobulin.

Methimazole vs PTU

PTU also blocks peripheral T4-to-T3 conversion, methimazole is more potent.

New Hormone Production Nearly Halted

Existing hormone stores are used up while new synthesis stays blocked.

• ~70%: TPO blockade (enzyme mostly inhibited)
• Depleting: Stored hormone (colloid reserve falling)
• Minimal: New synthesis (trickle output only)
• Weeks: Clinical lag (before levels drop)

Why effect is delayed

Stored hormone must be depleted before blood levels start falling.

Dose dependence

Higher dose deepens blockade and speeds hormone-store depletion.

T3/T4 Trending Toward Normal

Continued blockade steadily lowers circulating hormone over several weeks.

• 4–8 wks: Timeframe (typical normalization)
• Labs q4wk: Monitoring (T3, T4, TSH checked)
• Ongoing: Dose titration (adjusted to response)
• Improving: Symptom relief (as levels fall)

Tracking the curve

Hormone levels decline gradually, not immediately, over weeks of therapy.

Adjusting treatment

Dose lowered once levels approach the normal reference range.

Normalization and Agranulocytosis Vigilance

Hormones stabilize with therapy; rare severe WBC drop needs monitoring.

• ~0.3%: Agranulocytosis risk (rare but serious)
• Fever, sore throat: Warning signs (seek care immediately)
• 4500–11000: Normal WBC (cells/µL)
• Stop drug: Action if low (urgent CBC needed)

Successful outcome

T3/T4 reach normal range with continued, monitored thionamide therapy.

Agranulocytosis warning

Sudden fever or sore throat warrants immediate white cell count check.