Malabsorbtion vs pseudo-malabsorption in levothyroxine absorption test

Abstract

Introduction: The most common cause for persistent elevation of TSH levels in hypothyroid patients treated with levothyroxine is poor compliance. The Levothyroxine Absorption Test (LAT) is usually confirmed this phenomenon called 'pseudo-malabsorption'. Case report: 60-year-old female, weight 60kg, BMI 24.3kg/m2, presented with sleepiness, tiredness, fatigue and forgetfulness. Her skin was very dry and flaky. She had low tolerance of effort, poor appetite with weight oscillation around 2kg, constipation and sometimes heartburn. Hypothyroid for ten years after radioiodine treatment of Graves' disease. Her TSH levels were higher than normal, TSH 20-70mIU/L, in spite of efforts to adjust the dose (different LT4 preparations). In last two years her daily LT4 dose was 900 (15 µg/kg), 3x300mcg, 500+400mcg. Comorbidities: depression, angina, hypertension, absolutely arrhythmias (with inadequate INR in last two months). Before testing TSH 33.6mIU/L, FT4 4.25 pmol/L. Standard (1000 µg) LAT was performed under supervision. TSH, T4 and FT4 were measured 2h, 4h, 6h and 24h upon LT4 administration. Baseline values were TSH 26.92 mIU/L; FT4 4.4pmol/L; T4 41.5pmol/L. The lack of TSH fall with slight T4 and FT4 increase, significantly below expected AUC, pointed an inadequate absorption. It was started with 300 µg LT4 oral suspension, after testing. The presence of fat in the stool and positive antiparietal antibodies increased suspicion to malabsorptiv syndrome. EGDS was performed, PH finding confirmed H. pylori positive chronic atrophic gastritis, without morphological elements for GSE. Eradication treatment and IPP were introduced. After four weeks her thyroid hormones were TSH 1.63 mIU/L; FT4 26.6pmol/L, FT3 3.87pmol/L. Conclusion: LAT is useful for identifying much rare malabsorption. The adequate treatment lead to proper substitution and avoidance of no rationale increase of levothyroxine dose.