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Thyroid

(22.06) Levothyroxine Absorption Test: A Limitation in the Diagnosis of Delayed Absorption

Saturday, May 11, 2024
12:50 PM – 1:05 PM CDT
Location: ePoster Showcase, Empire Ballroom Foyer, Level 2
Introduction : The oral levothyroxine absorption test is an infrequently used test that measures intestinal absorption of free T4 to diagnose malabsorption as the cause of persistent, uncontrolled hypothyroidism. We present a case of a patient with a positive result of a 1000 mcg levothyroxine test at 2 hours, indicating malabsorption, who was then later found to have a normal response at 16 hours, calling into question the positive predictive value of the test.

Case(s) Description :

A 48-year-old woman was admitted in February 2023 for upper respiratory infection symptoms and was found to have a significant pericardial effusion. She had a long-standing history of hypothyroidism managed with a 250 mcg dose of levothyroxine but had a consistently undetectable free T4 between < 0.10 and 0.68 ng/dL with a persistently elevated thyroid stimulating hormone (TSH) between 10.6 and 37.6 uIU/mL for the past 4 years despite her claims of taking the medication as prescribed. There was suspicion of malabsorption and a Celiac’s disease antibody panel was ordered showing a moderately elevated deamidated gliadin IgA antibody titer; however, biopsy later was negative for Celiac’s disease. This was followed by a levothyroxine absorption test which showed an increase in her free T4 of less than 50% from 0.46 to 0.53 ng/dL 2 hours after administration of 1000 mcg of levothyroxine orally, suggesting malabsorption. However, the next morning, approximately 16 hours after levothyroxine administration, her free T4 was in the normal range at 1.02 ng/dL, indicating delayed absorption and possible medication non-adherence. She is currently being managed on 300 mcg of levothyroxine orally with free T4 in the normal range.

Discussion :

Our case highlights a potential limitation of the levothyroxine absorption test; based on the current cutoff, our patient would have classified as malabsorption while further testing indicates it was due to delayed absorption. We propose that the 2 hour test be followed by a later free T4 measurement at either 12 or 24 hours which may help to improve the efficacy of the test.