Optimal preparation for I-131 scanning and therapy of differentiated thyroid carcinoma requires use of a low iodine diet (< or = 50 micrograms iodine per day) to enhance the delivery of I-131 to thyroid remnants or carcinoma. Because there are no commercial low-iodine tube-feeding diets, the authors formulated and tested a suitable diet that is appropriate for home preparation using commonly available ingredients. A patient was placed on this diet in anticipation of radioiodine therapy after surgical resection of a thyroid carcinoma recurrence in his neck. This patient reduced his 24-hour urinary iodine excretion from 378 micrograms iodine to 13 micrograms iodine after 15 days on the diet. Analysis of the diet reveals that it is nutritionally complete and delivers only 18.2 micrograms of iodine for each 1000 kcal. This diet should improve I-131 therapy of differentiated thyroid carcinoma in patients requiring tube feedings.