Thyroxine replacement in post-radioiodine hypothyroidism.

Thyroxine replacement dose in 70 patients with post-radioiodine (for Graves' thyrotoxicosis) hypothyroidism was compared with that in 34 patients with autoimmune hypothyroidism matched for age and sex and diagnosed during the same period. Median replacement dose in the post-radioiodine group (100 micrograms daily) was significantly lower (P = 0.006) than in the autoimmune hypothyroid group (137.5 micrograms daily). Furthermore there was marked variation in the time elapsing from onset of post-radioiodine hypothyroidism to achievement of a given maintenance dose. These findings may be explained by persistence of non-TSH mediated thyroid function, after radioiodine therapy for Graves' thyrotoxicosis. The observations indicate the potential risk of overtreatment with thyroxine in post-radioiodine hypothyroidism unless the dose is carefully titrated against serum TSH levels. Final thyroxine requirements may not be evident in the initial phase of treatment and patients with apparently low maintenance thyroxine requirements require long-term follow up.