Fragmento En el control y seguimiento de pacientes tratados con yodo radioactivo (131I) por enfermedad de Graves y bocio modular tóxico, la evaluación del estado funcional de la glándula tiroides es importante porque hay una alta incidencia de hipotiroidismo, y en algunos una recurrencia del hipertiroidismo. La mayor incidencia de hipotiroidismo se presenta en los primeros años después del tratamiento. sin embargo la posibilidad de desarrollar hipotiroidismo puede presentarse con el transcurso de los años. ABSTRACT: 68 patients treated with 131I-iodide for Grave's and Plummer's Disease in Cali, Colombia were studied. TT4. PBI, T3 uptake, TBG and TSH were used to define their thyroid state. Serum T3 measurements (Radio Immunoassay) were performed in all and the importance to define the thyroid state was studied. A combination of TT4 and T3 uptake was helpful to classify the thyroid state in 81% of the patients. The remainder (19%) need additional tests (T3, TBG, TSH) for a correct classification. Two patients with clinical hyperthyroidism and normal serum TT4 had increased serum T3. The inappropriately normal TT4 value was not explained by deficiency of T4-binding globulin. One patient, clinically euthyroid, was maintained euthyroid by preferential secretion of T3 in the presence of low T4. 60% of the hypothyroid patients had serum T3 values in the normal range. These findings support the concept that TSH hypersecretion may result in T3 release despite diminished T4 secretions after radiation treatment. Measurements of serum T3 is useful in certain well-defined situations.