Development of hyperthyroidism in nodular goiter and thyroid malignancies in an area of relatively low iodine intake.

A high incidence of toxic nodular goiter has recently been described in areas of relatively low iodine intake. We studied the development of hyperthyroidism in nodular goiter and thyroid malignancies in an area of relatively low iodine intake (median 87 micrograms/h). The material comprised a total of 557 patients admitted to our department in the period 1978-88. The follow-up material consisted of 214 patients with a median age of 55 yr (19-86). Eighty-eight percent were females. The period of observation was 33 months (1-205). All patients had technetium thyroid scans, serum T3, serum T4, T3 uptake test and serum TSH performed. TRH tests were performed in 58 patients. During the follow-up period 45 initially euthyroid patients became hyperthyroid (18%). The incidence increased with age, and the median age in the group, who developed hyperthyroidism, was 65 yr. An estimated minimal incidence of toxic nodular goiter was 23/100,000/yr. Contrary to previous assumption, hyperthyroidism developed frequently in elderly patients with nodular goiter in a low iodine intake area. Thyroid cancer was found in 13 patients corresponding an estimated incidence of 1.5/100,000/yr. The cancer incidence was very low compared to other nordic countries with high iodine intake. This finding may suggest a possible influence of iodine intake on the incidence of thyroid cancer.