Laboratory Testing Before Propylthiouracil (PTU) Therapy
Before initiating PTU therapy, obtain a complete blood count (CBC) with differential and baseline liver function tests (LFTs) including transaminases and bilirubin.
Essential Baseline Laboratory Tests
Complete Blood Count (CBC) with Differential
- Obtain baseline CBC to establish pre-treatment white blood cell count and absolute neutrophil count 1
- This is critical because agranulocytosis is a serious potential adverse effect of PTU, though the incidence is similar between PTU and methimazole 1
- Baseline values allow for comparison if leukopenia or agranulocytosis develops during treatment
Liver Function Tests (LFTs)
- Measure baseline transaminases (ALT/AST) and bilirubin before starting PTU 1
- PTU carries significantly higher risk of hepatotoxicity compared to methimazole, with odds ratio of 2.40 for liver function injury and 3.96 for elevated transaminases 1
- Baseline LFTs are essential because PTU can cause serious hepatic complications including hepatitis and cholestatic jaundice 2
Additional Considerations
ANCA Testing (Not Routinely Required at Baseline)
- While PTU can cause ANCA-positive vasculitis with severe multisystemic involvement including pericardial, dermal, and joint manifestations 2, baseline ANCA testing is not standard practice
- Consider baseline ANCA only in patients with pre-existing autoimmune conditions or vasculitis symptoms
Thyroid Function Tests
- Baseline TSH, free T4, and free T3 should be obtained to document the degree of hyperthyroidism and guide dosing, though this is for disease assessment rather than drug safety monitoring
Common Pitfall to Avoid
Do not skip baseline liver function testing - PTU has substantially greater hepatotoxic potential than methimazole 1, and establishing baseline values is essential for detecting drug-induced liver injury during treatment monitoring.