Endocrine Abstracts

Introduction: Despite being on adequate amounts of levothyroxine (>1.6 μg/kg) some patients still exhibit biochemical evidence of inadequate replacement (serum TSH >4.2 mIU/l±free-thyroxine (FT₄) <12 pmol/l). We report the use of a 5-day absorption test for assessing levothyroxine absorption in such a patient.

Case: A 35-year-old female with hypothyroidism since 2007 had a serum TSH ranging from 4.48 to 54.9 mIU/l and FT₄ ranging from 5.5 to 8.9 pmol/l, despite being on levothyroxine (150–400 μg/day) for several months. She had iron deficiency anaemia but normal serum vitamin B12, folate, calcium and anti-TTG antibody levels, and normal gastro-endoscopic examination. After an unsuccessful six-week trial of the patient self-administering her levothyroxine tablets in the morning on an empty stomach, we therefore performed a supervised 5-day levothyroxine absorption test using her current dose (150 μg/day).

Methods: The patient attended the endocrine unit every morning (Monday to Friday) for supervised administration of her levothyroxine tablets on an empty stomach. Blood samples for thyroid hormones were taken pre-dose on day-1 and 2 h post-dose on day-5.

Results: The 5 day absorption test improved the thyroid hormone profile in this patient by the fifth day. On day-1 the pre-dose serum TSH was 27.9 mIU/l and the FT₄ was 11.3 pmol/l, while on day-5 the 2-h post-dose TSH was 1.9 mIU/l and the FT₄ was 34.6 pmol/l. These results indicate that continued daily supervised administration of levothyroxine (150 μg/day in this case) would have resulted in significant drug-induced thyrotoxicosis.

Conclusion: The supervised 5-day (usual dose) levothyroxine test is as useful as the 1-day (1000 μg) test for assessing levothyroxine absorption in patients with apparent malabsorption or pseudomalabsorption of levothyroxine, and may be more appropriate for patients who take less than 500 μg/day of levothyroxine. Furthermore, this can be achieved in an outpatient setting. Further studies are required to validate and standardise the 5-day absorption test.