Diffusely reduced 99Tcm-pertechnetate uptake is a relatively infrequent but annoying finding that impairs evaluation of the thyroid gland. We studied 32 female patients aged 19–85 years with markedly reduced pertechnetate uptake. The following causes of reduced pertechnetate uptake were recognized: treatment with iodinated pharmaceuticals (n = 15), suppression therapy with T4 (n = 11), subacute thyroiditis (n = 5) and massive tumour replacement (n = 1). 99Tcm-MIBI thyroid scintigraphy was performed within 24 h of the pertechnetate study. The results were correlated with neck ultrasound, serum TSH (n = 25) and surgical findings in patients who had been operated on. The technique identified the following conditions: normally sized thyroids (n = 4), diffuse goitres (n = 8), multinodular goitres (n = 17) and solitary thyroid nodules (n = 3). Moreover, substernal goitres were identified in nine patients. This condition was confirmed at surgery in seven patients. Ultrasonography was concordant in 29 of 32 patients in terms of thyroid size and structure, but failed to demonstrate substernal thyroid tissue. Our results suggest that 99Tcm-MIBI scintigraphy may contribute to the diagnosis of thyroid pathology and treatment planning in patients with diffusely decreased 99Tcm-pertechnetate uptake.