Nonadherence to levothyroxine therapy is one cause of persistent hypothyroidism. To distinguish nonadherence from malabsorption, a levothyroxine absorption test (LAT) is sometimes required. The authors report a case of a 60-year-old female patient suffering from hypothyroidism resistant to oral levothyroxine (LT4) substitution after radioiodine therapy for Graves disease. Despite the continuous increases of LT4, over 10 years, to a maximum of 1 mg/day, she 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 min after 1mg of LT4. The results are presented in the graphic below. The patient remained with normal heart rate and normotensive along LAT. The results showed a normal absorption of LT4, so pseudo-malabsorption was proven. Based on this diagnosis was prescribed 1 mg 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, secure and useful tool in this cases.
20 - 23 May 2017
European Society of Endocrinology