Subcutaneous Levothyroxine Administration in Treatment of Refractory Primary Hypothyroidism: A Case Report

Background: We assessed the effectiveness of administering subcutaneous levothyroxine in a medically complex patient, a 51-year-old male who previously underwent total thyroidectomy for papillary thyroid carcinoma. His thyrotropin (TSH) worsened to >100 mIU/L, caused by encapsulating sclerosing peritonitis that led to thyroxine malabsorption and enteral loss of protein-bound thyroxine. Several routes of levothyroxine were evaluated prior to subcutaneous levothyroxine. Methods: Subcutaneous levothyroxine was initiated at a low dose of 100 mcg thrice a week. A pharmacokinetic absorption study was performed to assess the bioavailability of subcutaneous levothyroxine against oral levothyroxine. Results: A 103% increase in free thyroxine at 6 hours post-subcutaneous levothyroxine 100 mcg confirmed effective absorption. Area-under-curve analysis showed that the relative bioavailability of subcutaneous levothyroxine was 8.75 times of oral levothyroxine for our patient. Hence, subcutaneous levothyroxine was initiated, with TSH normalizing 20 days later. Conclusions: Subcutaneous levothyroxine may be an alternative in patients with oral malabsorption.