Thyrotoxic Encephalopathy With Seronegative Thyroiditis: An Unusual Occurrence

Thyrotoxic encephalopathy is a rare phenomenon that is associated with autoimmune thyroid disorders (Hashimoto's thyroiditis or Graves' disease), where elevated levels of antithyroid antibodies with an excellent response to steroids are seen. Here, we describe a case of thyrotoxic encephalopathy due to seronegative thyroiditis. There are no reports of encephalopathy occurring in a patient with thyrotoxicosis in the absence of thyroid antibodies in the English literature. A 50-year-old woman who was admitted with fever and subclinical hyperthyroidism with negative antibodies progressed over a period of 10 days to develop acute confusion and frank hyperthyroidism. Treatment was started with carbimazole; however, following negative antibodies and imaging studies in keeping with thyroiditis, carbimazole was stopped. Her symptoms worsened, and thyroid function tests continue to show hyperthyroidism. She was extensively investigated for encephalopathy by a multidisciplinary team comprising an endocrinologist, a neurologist, and an infectious diseases team, and neurological causes were comprehensively ruled out. She was restarted on carbimazole, and over the next seven days, her confusion settled, and she returned to her baseline functioning. A diagnosis of thyrotoxicosis secondary to seronegative thyroiditis was made. Over the subsequent few weeks, carbimazole was withdrawn, and she was started on levothyroxine. She continued to make good progress. We present the first case of seronegative thyrotoxicosis and a review of the literature. It is important to consider thyrotoxicosis as a cause for unexplained encephalopathy and to remember that it can be seen with seronegative thyroiditis. It is a diagnosis of exclusion with treatment involving antithyroid drugs.