Levothyroxine remains the standard therapy for hypothyroidism, and true intolerance to this medication is considered exceedingly rare. This report describes the case of a young female who developed nonspecific gastrointestinal and respiratory symptoms, initially attributed to levothyroxine intolerance. The subsequent occurrence of similar symptoms among family members, together with isolated eosinophilia identified on complete blood count, prompted consideration of an alternative aetiology. Parasitological evaluation confirmed infection with Ascaris lumbricoides. Following antiparasitic therapy, the patient’s symptoms resolved fully. This case underscores the critical importance of comprehensive clinical assessment and differential diagnosis in patients presenting with atypical symptoms during levothyroxine therapy. Given the rarity of true levothyroxine intolerance, clinicians should thoroughly evaluate for alternative causes to avoid diagnostic error and inappropriate treatment modification.