Baseline Graves' Hyperthyroidism
Autoantibodies overdrive the thyroid, flooding the body with hormone.
- 4.6: Free T4 (baseline) (ng/dL, markedly high)
- 100%: TPO activity (unchecked iodination)
- Suppressed: TSH (negative feedback)
- ~1%: Prevalence (lifetime, women > men)
TSH-receptor antibodies
Autoantibodies mimic TSH, continuously stimulating the thyroid gland.
TPO-driven iodination
Thyroid peroxidase oxidizes iodide and attaches it to thyroglobulin tyrosines.
Clinical picture
Tachycardia, weight loss, heat intolerance, and tremor result from excess hormone.
Untreated Graves' disease can progress to thyroid storm, a medical emergency.
Antithyroid Drug Administered
Thionamide molecules enter the follicle and seek out TPO enzymes.
- Thionamide: Drug class (methimazole / PTU)
- TPO enzyme: Target (active-site inhibitor)
- Faster: Onset (methimazole) (longer half-life)
- Slower: Onset (PTU) (also blocks T4→T3)
Mechanism of inhibition
Drug molecules are oxidized by TPO, consuming the enzyme's catalytic capacity.
Dose dependence
Higher doses saturate more TPO molecules, blocking hormone synthesis faster.
Methimazole vs PTU
PTU also blocks peripheral T4-to-T3 conversion; methimazole is more potent overall.
Hormone Synthesis Progressively Blocked
With TPO inhibited, stored hormone depletes and free T4 declines over weeks.
- 6–8 wks: Time to euthyroid (typical course)
- ~7 days: T4 half-life (gradual decline)
- Depleting: Colloid store (no new hormone made)
- Ongoing: Titration (dose adjusted to labs)
Depleting the reservoir
Existing stored hormone in colloid is released even as new synthesis halts.
Lab-guided titration
Free T4 and TSH are rechecked periodically to fine-tune the dose.
Symptom resolution
Heart rate, tremor, and weight trends normalize as hormone falls.
Agranulocytosis Surveillance
Routine blood counts watch for a rare but dangerous neutrophil crash.
- ~0.3%: Agranulocytosis risk (of treated patients)
- <500: Danger threshold (neutrophils/µL)
- First 90 days: Onset window (highest risk period)
- Stop drug: Action if crossed (immediate discontinuation)
What to watch for
Sore throat and fever are the classic warning symptoms to report urgently.
Dose-risk relationship
Higher doses correlate with steeper, faster neutrophil declines.
Response protocol
Drug is stopped immediately and counts are monitored until recovery.
A neutrophil count under 500/µL is a medical emergency requiring drug cessation.
Outcome — Euthyroid State or Escalation
Most patients stabilize on therapy; some need definitive treatment instead.
- ~40-50%: Remission rate (after 12-18 month course)
- ~50%: Relapse rate (after stopping drug)
- RAI / surgery: Alternative (if adverse effects)
- Euthyroid: Goal (normal T4 and TSH)
Successful maintenance
Continued low-dose therapy keeps hormone levels in the normal range.
When to escalate
Agranulocytosis or poor control prompts a switch to RAI or surgery.
Long-term outlook
Many patients eventually taper off with sustained remission.
Definitive treatment options