A comparison of 99mTc and 123I scintigraphy in nodular thyroid disorders

Out of 450 patients referred for thyroid scintigraphy 83 consecutive patients with nodular goitre were selected for dual scintigraphy with ^99mTc and ¹²³I. Seventy-four patients had one or more palpable nodules and the other nine patients had palpably diffuse goitres with one or more nodules on the routine ^99mTc scintigram. For the comparison of scintigrams an eight level colour display was used, and regional differences between the ^99mTc and ¹²³I scintigrams were defined as a minimum of 25% (2 colour levels) variation of the relative activity in well defined areas.Thirteen patients, i.e. 16% of those with nodular goitre, had discordant scintigrams. All 13 patients had non-malignant thyroid disorders, (simple goitre, radio-iodine treated hyperthyroidism, autonomous adenoma, subacute thyroiditis, Riedel's and lymphocytic thyroiditis). The diagnosis was verified histologically in eight of these patients.Eight of the 13 patients had a high ^99mTc activity relative to ¹²³I, five patients showed the opposite discrepancy. Another 5 patients with a follicular or mixed follicular thyroid carcinoma had identical scintigrams.These findings indicate that ¹²³I does not offer such diagnostic advantages over the cheap and readily available ^99mTc, that its use for routine scintigraphy of the thyroid gland seems to be justified at present.