ESPE Abstracts

Hypothyroid patients post-thyroidectomy require 1.6-1.8 mg/kg/die of Levothyroxine (L-T4). A persistent TSH level above the upper limit despite a daily dose >1.9 mg/kg/die is defined as “refractory” hypothyroidism in adults. Malabsorption and “pseudo-malabsorption” need to be investigated in case of therapeutic failure. L-T4 absorption test (LT4AT) is useful in distinguishing these two conditions but it has only been described in adult age. We present the case of a 14-year-old girl affected by severe refractory hypothyroidism after thyroidectomy, despite apparently taking high dosage of L-T4. At 10 years of age she was diagnosed with autoimmune hyperthyroidism (FT4>6 ng/dl, FT3>7 pg/ml, TSH<0,01 mcU/ml, TRAb>40 U/l) and she started treatment with Methimazole (0.5 mg/kg/day). After 15 months of anti-thyroid drug, the patient still presented not well-controlled thyroid function values and an important thyromegaly. Therefore, at 12 years of age she underwent total thyroidectomy. The postoperative period was regular, except for transient hypocalcemia requiring calcium and vitamin D. L-T4 therapy was started at a dosage of 1.5 mg/kg/day. One year after surgery, TSH values oscillated between 25 and 120 μcU/ml with low FT4 despite the treatment with progressively higher dosage of L-T4 (4 mg/kg/day) and referred adequate compliance. Therefore, she was hospitalized in our pediatric department. Thyroid function test was performed at the entrance and after five days of L-T4 administration under the nurses' supervision. All causes of malabsorption were ruled out as well as medications impairing L-T4 absorption. Thus, we decided to perform LT4AT test. We administered L-T4 1000 mg peroral and collected blood tests (TSH, FT4, FT3) at baseline and at 1-2-3-4-6-24 hours after drug intake. The concentrations of TSH, FT3 and FT4 at different timepoints are reviewed in the following table. The test showed a >2 time increase in basal FT4 levels and a normalization of the thyroid function at seven days, excluding L-T4 malabsorption. We suggested to restart L-T4 treatment under parents’ strict surveillance with close monitoring of thyroid function tests. A possible non-adherence to L-T4 therapy must be investigated while managing refractory hypothyroidism and LT4AT test could provide valuable information also in adolescence.