Impaired Enteral Levothyroxine Absorption in Hypothyroidism Refractory to Oral Therapy After Thyroid Ablation for Papillary Thyroid Cancer: Case Report and Kinetic Studies

Abstract

We present a 49-year-old patient suffering from hypothyroidism (thyrotropin [TSH], 20–80 mU/L) refractory to oral levothyroxine (LT₄) substitution after total thyroidectomy and radioiodine therapy for papillary thyroid cancer. Extensive, repetitive work-up excluded small bowel, liver, and pancreatic disease and drug interactions. Triiodothyronine/thyroxine (T₃/T₄) antibodies were negative and reverse T₃ levels were normal. Supervised absorption tests of several LT₄ preparations in increasing dosages (250–1500 µg/d) confirmed an insufficient rise in serum T₄ levels following oral drug administration. Thus, intravenous LT₄ application (5 times per week) was commenced to restore at least normal range TSH levels. Repetition of absorption tests 1 year later, after a documented period of more than 3 months of stable euthyroidism, showed a considerable improvement of intestinal LT₄ uptake. Subsequently, the patient was managed on large doses of oral LT₄ (1500–2100 µg/d) alone, but this was found to be insufficient so that continuous intravenous LT₄ was administered via a morphine pump device to ultimately achieve stable euthyroidism. Of note, at 4-year follow-up there was no evidence of recurrent or metastasized papillary thyroid cancer. The reason for the disturbed intestinal LT₄ absorption in this patient remains unresolved. However, we would like to suggest the possibility of a specific intestinal uptake deficit, which will only become apparent in the case of previous thyroid ablation.