Thyroxine intoxication is a benign, nonfatal condition, relatively common in the pediatric age group. We present here a detailed laboratory follow-up of all thyroidal hormones in 2 healthy girls who inadvertently ingested 2,500 μg of L-thyroxine. The two girls were hospitalized and treated with ipecac, gastric lavage, propranolol, prednisone, cholestyramin and propyl-thiouracil. All physical signs were normal and no symptoms were reported. All thyroidal hormones were measured 12 times from 2 h to 20 days after the ingestion. For T<sub>4</sub>, T<sub>3</sub>, rT<sub>3</sub> and thyroglobulin (Tg) a one-compartment kinetic model was formulated and fitted to the empirical data. The kinetic data constants of production and elimination were calculated, as well as the metabolic clearance rate. All laboratory values were similar in both girls. T<sub>4</sub> serum levels were already high 2 h after the intoxication and returned to normal values only after 13 days. Fitting the T<sub>4</sub> serum levels with a one-compartment model resulted in absorption and degradation constants similar to those in normal adult subjects. Thyroid-stimulating hormone (TSH) levels decreased reaching their lowest concentration 14 h after the intoxication. They remained low till the 4th day, after which they rose gradually. Twenty days after the intoxication, TSH levels were still below their initial values. T<sub>3</sub> reached its peak levels 11 h after the ingestion and decreased to normal values after 3 days. Both T<sub>3</sub> production constants and T<sub>3</sub> degradation constants were significantly increased. rT<sub>3</sub> reached its peak level on the 2nd day after the intoxication and decreased to normal values on the 4th day. Its production and degradation constants were somewhat below normal levels. The T<sub>3</sub>/rT<sub>3</sub> ratio decreased from a normal level of around 3 to as low as 1 and rose again after 13 days to extremely high levels (as high as 8). Tg serum levels dropped continuously with a half-life of 1-5 days and started rising again after 2-13 days. In conclusion, T<sub>4</sub> intoxication in the child is combated primarily by a significant increase in T<sub>3</sub> production and degradation, while meticulously maintaining relatively low T<sub>3</sub> levels.