PTU Dose in Thyroid Storm
Administer propylthiouracil (PTU) at 200 mg orally every 4-6 hours (800-1200 mg/day) as the preferred first-line thionamide for thyroid storm, given its dual mechanism of blocking both thyroid hormone synthesis and peripheral T4 to T3 conversion. 1
Why PTU is Preferred Over Methimazole
PTU has a unique advantage in thyroid storm because it inhibits both thyroid hormone synthesis AND peripheral conversion of T4 to T3, making it the first-choice thionamide according to European Society of Cardiology guidelines 1
Research demonstrates that PTU produces a significantly greater acute decrease in serum T3 levels compared to methimazole when combined with iodide therapy - T3 dropped to 326 ng/100 ml on day 1 with PTU versus 568 ng/100 ml with methimazole 2
The serum T4/T3 ratio increased to 88-91 during days 3-5 with PTU treatment compared to only 52-54 with methimazole, confirming PTU's superior blockade of peripheral T3 production 2
Dosing Algorithm
Initial Loading Phase
- Start PTU at 200 mg orally every 4-6 hours (total daily dose 800-1200 mg/day) 3
- Methimazole 20-40 mg orally every 6-8 hours can be used as an alternative if PTU is unavailable 1
Critical Timing with Iodine
- Administer saturated potassium iodide solution or sodium iodide 1-2 hours AFTER starting PTU, never before, as giving iodine first can worsen thyrotoxicosis 1
Maintenance Dosing
- Continue full thionamide doses until free T4 and T3 levels approach the upper limit of normal 4, 5
- Once thyroid hormones normalize, reduce PTU dose by 30-50% with monitoring of thyroid function every 2-3 weeks 5
- Typical maintenance doses range from 150-200 mg/day of PTU 3, 6
Monitoring Requirements
Check free T4 and T3 levels every 2-3 weeks during the tapering phase to guide dose adjustments 4, 1
Monitor for agranulocytosis with complete blood counts, particularly during the first 3 months of therapy 1
Watch for acute hepatitis, which is a rare but potentially fatal complication of PTU - monitor liver function tests (ALT, bilirubin, alkaline phosphatase) regularly 3
Critical Pitfalls to Avoid
Never administer iodine before thionamides - this can paradoxically worsen thyrotoxicosis by providing substrate for additional hormone synthesis 1
Do not taper PTU too early - maintain full doses until thyroid hormones approach normal range, as premature reduction increases relapse risk 4, 5
Be vigilant for hepatotoxicity - a case report documented ALT rising to 852 U/L with bilirubin of 46 μmol/L requiring immediate PTU discontinuation and supportive care 3
TSH may remain suppressed for weeks to months after clinical improvement, so do not rely solely on TSH for dose adjustments 5
Special Considerations
Pregnancy
- PTU use should be restricted to the first trimester of pregnancy at doses of 150-200 mg/day, then switch to methimazole in the second trimester 6