Although headache attributed to hypothyroidism is coded within The International Classification of Headache Disorders, 3rd edition, an association between headache and thyrotoxicosis (hyperthyroidism) is mentioned only in the appendix. Reports on relevant cases are too scarce to establish a causal relationship. A young man with a history of migraine with aura arrived at our headache clinic with a 10-day history of headache and weight loss. Brain MRI revealed normal findings. Blood tests revealed thyrotoxicosis. A test for thyroid-related antibodies was negative. Thus, the patient was diagnosed with painless thyroiditis. The patient's headache resolved as his thyroid hormone levels decreased. To the best of our knowledge, this is the first reported case of headache exaggerated by painless thyrotoxicosis.