The aim of this paper was to compare ultrasound (US) assessment of thyroid volume with that obtained by positron emission tomography (PET), in patients scheduled for adaptive radioiodine therapy, in which 50 Gy was prescribed to the functional PET volume. Firstly a pilot study was performed to ascertain the optimum method for US assessment of thyroid volume. Then 17 comparative measurements of thyroid volume by US and PET were made on 15 patients (two male and thirteen female, ages 28-73 years) with suspected Graves' disease. This comparison showed that in normal sized and enlarged thyroid glands (n=13), the ratio of functional PET to anatomical US volume was approximately 2:3. However, using the same ellipsoid model, PET and US assessment of anatomical volume agreed within the measurement errors. Owing to the presence of nodules and non-uniform distribution of radioiodine, the functional PET volume and anatomical US volume are often not equivalent. If high-resolution emission tomography (e.g. PET) is unavailable, the comparative data presented in this paper could be used to derive the functional volume from the US volume for calculating functional thyroid dose in hyperthyroid patients undergoing radioiodine therapy.