A patient on a longer MDR-TB regimen containing ethionamide and para-aminosalicylic acid develops fatigue, cold intolerance, bradycardia and raised TSH after several months. The most likely explanation is:

• A Additive antithyroid effect of ethionamide combined with PAS ✓
• B Ethambutol induced thyroiditis
• C Cycloserine induced hypothalamic dysfunction
• D Moxifloxacin interference with thyroxine absorption

Explanation

Ethionamide (and prothionamide) impair thyroid hormone synthesis through blockade of iodine organification, an effect structurally related to its thiourea moiety. When combined with para-aminosalicylic acid, which also has weak antithyroid activity, the risk of hypothyroidism becomes clinically significant, so periodic TSH monitoring is recommended. No other listed anti-TB drug produces this endocrine toxicity.

Reference: Goodman and Gilman's The Pharmacological Basis of Therapeutics, 13th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.