Late and transient increases in free T4 after radioiodine treatment for Graves' disease.

The objective of this retrospective study was to evaluate the fall in free T4 (FT4) in patients with Graves' disease after treatment with radioiodine in a fixed dose of 600 MBq. The study was performed at our outpatient clinic with patients referred from primary care during the time period January 1989 to January 1995. Only patients not given anti thyroid drugs after radioiodine were included. FT4 and TSH were measured every second week for the first three months, and thyroxine substitution started when the FT4 was at or below 15 pmol/l. Of the 60 patients thus available for evaluation, 7 required retreatment, giving a "success rate" of 88%. Of the 53 patients successfully treated with one dose of radioiodine, 36 had not been pretreated with anti thyroid drugs. Among these patients 13 (36%) had a transient increase in FT4 after radioiodine therapy, which mostly occurred after 4 to 6 weeks. The remaining 17 patients had been given carbimazole prior to radioiodine. In this group 8 (53%) had a transient increase in FT4, generally after 2 weeks. In conclusion, giving a fixed large dose of radioiodine and starting thyroxine substitution before hypothyroidism has developed is a workable clinical routine. Although a gradual fall in FT4 was the rule, a transient increase in FT4 was noticed in 30-50% of the patients 2 to 6 weeks after treatment.