A patient on a second-line regimen containing both ethionamide and para-aminosalicylic acid develops fatigue, cold intolerance, weight gain, and a raised TSH after several months. What is the most likely explanation?
- A Ethionamide-induced inhibition of thyroid hormone synthesis, compounded by PAS ✓
- B Rifabutin-induced hypermetabolism masking hypothyroidism
- C Pyrazinamide-induced SIADH causing dilutional changes
- D Linezolid-induced autoimmune thyroiditis
Correct answer: A. Ethionamide-induced inhibition of thyroid hormone synthesis, compounded by PAS
Explanation
Ethionamide impairs thyroid hormone synthesis through interference with iodine incorporation, and the risk of hypothyroidism is markedly increased when it is combined with para-aminosalicylic acid. Monitoring TSH during combined therapy is recommended. This endocrine interaction distinguishes ethionamide from isoniazid, whose main systemic toxicities are hepatic and neurological despite their structural similarity.
Reference: Katzung Basic and Clinical Pharmacology, 16th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.