Nonadherence to levothyroxine therapy is one cause of persistent hypothyroidism. To distinguish nonadherence from malabsorption, a levothyroxine absorption test (LAT) is required. The authors report a case of a 60-year-old patient suffering from hypothyroidism resistant to oral levothyroxine (LT4) substitution after radioiodine therapy for Graves’ disease. Despite the continuous increases of LT4, over 15 years, to a maximum of 1 mg/day, the patient remained with high thyrotropin (TSH) and low free thyroxine (FT4). Extensive investigation excluded disease of the small bowel, liver and pancreas as well us drugs interactions.
After careful consideration was decided to conduct a LAT. Serum levels of TSH, FT4 and free triiodothyronine (FT3) were drawn at 0, 60, 120, 180 e 240 minutes after 1000mcg of LT4. The serum FT4 level increased from 2,1 pmol/L to 19,9 pmol/L and the serum TSH level declined from 34 UI/mL to 26 UI/mL within 4 hours after administration of LT4. Serum FT3 level also increased from 2,3 pmol/L to 2,7 pmol/L. Heart rate and blood pressure remain stable.
The results showed a normal absorption of LT4 so pseudo-malabsorption was proven. Based on this diagnosis was prescribed 1000 mcg of LT4 weekly to improve therapeutic compliance. Nowadays, patient present normal TSH and FT4 serum levels.
In conclusion, pseudo-malabsorption is an important differential diagnosis in persistent hypothyroidism to achieve therapeutic success and LAT is a simple and useful tool in this cases.

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