A 40-year-old man on an MDR-TB regimen containing ethionamide together with PAS presents with weight gain, cold intolerance, bradycardia and a raised TSH with low free T4. The most likely cause is:

• A Linezolid-induced thyroiditis
• B Euthyroid sick syndrome from chronic infection
• C Rifampicin-induced accelerated thyroxine catabolism alone
• D Drug-induced primary hypothyroidism, an additive effect of ethionamide with PAS ✓

Explanation

Ethionamide impairs thyroid hormone synthesis, and when combined with PAS, which also interferes with iodine handling, the effect is additive, producing goitrous hypothyroidism requiring levothyroxine replacement while continuing therapy if possible. Euthyroid sick syndrome does not produce a genuinely raised TSH with low free T4 pattern, which rules out the closest distractor. Monitoring thyroid function is recommended when both agents are co-prescribed.

Reference: Katzung Basic and Clinical Pharmacology, 15th ed.

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Written and medically reviewed by the StethoPrep medical team.