Several case reports have illustrated a rare neurological manifestation, idiopathic intracranial hypertension (IIH), in patients with thyrotoxicosis. However, none were diagnosed with thyroiditis. We report the case of a patient with subacute thyroiditis who presented with severe intractable headache due to IIH. . A 36-year-old woman visited Lampang Hospital in February 2021 complaining of neck pain and progressive severe intractable headache. Her vital signs and neurological examination were normal. Thyroid examination revealed a single 1 cm right thyroid nodule. A computed tomography (CT) scan of her brain illustrated diffuse brain edema. However, CT angiography and venography of the brain did not show abnormalities. The opening pressure of the cerebrospinal fluid was elevated (27 cmHO). The free triiodothyronine level was 6.19 pg/mL, free thyroxine was 2.32 ng/dL, and thyroid-stimulating hormone was 0.0083 IU/mL. Anti-Tg was positive at a low titer, but anti-TPO was negative. TRAb was also negative. Methimazole and acetazolamide were prescribed and monitored. The symptoms resolved completely within 2 weeks of onset. Thyroid hormones had returned to normal by 8 weeks.