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

<b><i>Background:</i></b> 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. <b><i>Methods:</i></b> 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. <b><i>Results:</i></b> 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. <b><i>Conclusions:</i></b> Subcutaneous levothyroxine may be an alternative in patients with oral malabsorption.