DELAYED INTESTINAL-ABSORPTION OF LEVOTHYROXINE

We report four female patients with nodular goiter (in two of the four due to Hashimoto's thyroidits) and one male patient with frank hypothyroidism due to Hashimoto's thyroiditis in whom TSH-suppressive or replacement L-T-4 therapy failed to suppress or, respectively, normalize serum TSH. As is typical in our country, our patients took L-T-4 15-20 min before a light breakfast, Gastrointestinal or other diseases and drugs known to interfere with the intestinal absorption of L-T-4 were not the cause of this failure, The gastrointestinal absorption test of L-T-4 (1000 mu g) was performed in four patients; in three patients it revealed peculiar abnormalities in that (i) the absorption peak was >70% but occurred at 4 hr vs an average of 2 hr in 12 euthyroid controls (EC) and 3 hr in 10 primary hypothyroid controls (HC); (ii) 50% of the maximal absorption occurred at 110 min vs 45 min in EC and 50 min in HC; (iii) the maximal increment in T-4 absorption was between 90 and 120 min (+111%) vs between 30 and 60 min in EC (+312%) and HC (+354%), In sum, only the first part of the absorption curve of T-4 was shifted to the right (in three of the four women) and this shift was more pronounced and extended to the second part of the curve in the fourth patient; in this last patient absorption peak was 44% at 180 min, Based on these results, we obtained full suppression or normalization of TSH by postponing breakfast for at least 60 min after T-4 ingestion. The precise cause for the ''inertia'' in the early phase of T-4 absorption is not known, but since it was observed in two patients with euthyroid nodular goiter, it is not associated with hypothyroidism per se.