Abstract
SUMMARY: Rare patients who have both thyroid-stimulating hormone (TSH) receptor-stimulating and -blocking antibodies can develop 'pendulum swinging' thyroid dysfunction. A 9-year-old girl with Down syndrome was treated with carbimazole for Graves' disease. After 2 years of treatment, she became profoundly biochemically hypothyroid, and this persisted after carbimazole was discontinued. Low-dose L-thyroxine was commenced. This was subsequently also discontinued as biochemical hyperthyroidism developed. TSH receptor antibody bioassay identified both TSH receptor-stimulating and -blocking antibodies. Mild hyperthyroidism persisted and while consultations regarding definitive treatment were ongoing, medication was not recommenced. Thyroid function normalised spontaneously and she has remained euthyroid for the past 3 years. Previous reports have advised definitive treatment; however, our patient developed spontaneous remission which has been prolonged and definitive therapies have been avoided. It is not yet known how commonly this particular phenomenon occurs. LEARNING POINTS: Rare patients who have both TSH receptor-stimulating and -blocking antibodies can switch between hyperthyroidism and hypothyroidism or vice versa during treatment with antithyroid drugs or thyroxine. Metamorphic thyroid autoimmunity is more common in Down syndrome. Switching between hyperthyroidism and hypothyroidism and back again is less commonly reported. Definitive treatment such as radioactive iodine or thyroidectomy are usually recommended. Prolonged remission was achieved off all medication, without recourse to definitive treatments.