Endocrine Abstracts

Levothyroxine (L-T4) is the mainstay of treatment of hypothyroidism. Marked elevation of thyrotropin (TSH) in patients on high replacement doses is rare and can result from malabsorption, drug interaction or poor compliance. The levothyroxine absorption test is required to distinguish these causes and has proved to be safe. This test measures the serum free thyroxine (FT4) response to 1 mg of oral L-T4 over 4–24 h.

Clinical cases: Case no 1: A 28-year-old woman presented with congenital primary hypothyroidism, being supplemented since the neonatal period. She has no other relevant medical history and denies additional medications. Review of old records revealed marked elevation of TSH despite L-T4 supplementation at progressively higher doses. In October 2017 she had TSH 182 μUl/ml while taking 112 mcg daily of L-T4 (1.5 mcg/kg). The patient reported taking L-T4 every morning in the fasting state. We performed the L-T4 absorption test with 1 mg of L-T4: TSH 33,3 μUl/ml and FT4 1,01 ng/dl at baseline; TSH 32,1 μUl/ml and FT4 2,12 ng/dl after 4 h. This result confirmed the diagnosis of non-adherence.

Case no 2: A 36-year-old woman with primary hypothyroidism (antibody-negative thyroiditis) presented with uncontrolled hypothyroidism. She has history of Sjogren’s syndrome. Medications in addition to L-T4 include hydroxychloroquine, methotrexate and folic acid. In December 2017 she had TSH 58.5 μUl/ml while taking 375 mcg daily of L-T4 (5.3 mcg/kg). She reports taking the levothyroxine regularly in the fasting state. The possibility of a malabsorption state was considered. Autoimmunity for celiac disease was normal, but gastric parietal cell antibodies were positive. Vitamin B12 levels were normal. Although it is recognised that there may be an increased need for L-T4 in patients with atrophic gastritis, this patient is already taking a very high dose. We are going to perform a L-T4 absorption test to exclude non-adherence.

Conclusion: The levothyroxine absorption test allows to distinguish between malabsorption and non-adherence. This test may be particularly useful in cases where malabsorption and noncompliance are likely to explain the failure of therapy. Testing over 4 h offers a safe alternative to longer protocols.