Thyroxine toxicosis in patients with iodine induced thyrotoxicosis

Serum concentrations of thyroxine (T4) and triiodothyronine (T3) were measured in 74 untreated hyperthyroid patients. Of these, 47 had Graves' disease, 12 had toxic nodular goiter and 15 had iodine induced thyrotoxicosis (IIT). The average serum T4 concentrations were, respectively, 13.5 microgram/100 ml and 13.3 microgram/100 ml (normal 4.5-10.3 microgram/100 ml). The average serum T3 concentrations were in the same order 732 ng/100 ml, 505 ng/100 ml and 378 ng/100 ml )normal 116-288 ng/100 ml). Of the 15 patients with IIT 5 had a pattern of T4-toxicosis (T4 larger than or equal to 10.5 microgram/100 ml; T3 less than 290 ng/100 ml). This pattern was not found in any of the patients from the other two groups. In IIT the ratio between the serum concentrations of serum T4 and T3 is similar to the one observed in normal individuals from non endemic areas. In patients with Graves' disease and toxic nodular goiter the increase in serum T3 is twice as high as the increase in serum T4. It is concluded that in IIT the serum concentrations of T3 and T4 increase in parallel while in the other two groups there is a preferential secretion of T3. As a consequence, a pattern of T4 toxicosis is a likely event in IIT and, when present, may constitute a biochemical characteristic of the condition.