Introduction: The thyroxine absorption test (TAT) is well established to investigate persistently raised TSH in patients on L-thyroxine. We review our experience with this test.
Method: Blood was taken for baseline FT4, FT3, TSH measurements and malabsorption screen. A weeks supply of L-thyroxine (1.6 mcg/kg×7) was administered orally under direct supervision and FT4 and TSH measured 2 h later. Patients continued on the same weekly dose for 4 weeks and increased, if TSH still elevated, to 2 mcg/kg×7 for 2 weeks; thereafter the total dose was given in two divided doses, twice weekly. Absorption was considered adequate if FT4 increased by >50% compared to baseline value at 2 h. FT4 and TSH were measured by chemiluminescence on an Abbot analyser. TFT data was collected at ≥6 months following the test to review compliance.
Results: Twenty four patients (4 male, 20 female) with a mean (range) age of 36 (1674) years, and weight (range) 86 (47124) kg underwent TAT. Prior to testing, the mean daily L-thyroxine dose was 222 (range: 75375 (g/day). Adequate L-thyroxine absorption was demonstrated in 96% patients. In 71% of patients, TSH values normalised after 46 weeks confirming poor compliance. Seven patients failed to suppress TSH at the end of 46 weeks; one patient was on dialysis and TSH normalised following renal transplantation; three required 2.0×weight (kg)×7 g once weekly and a further three required higher dose split as twice weekly. By six months, TSH values were maintained in range or suppressed (as needed in high risk thyroid cancer) in 85% patients.
Conclusion: TAT is useful where non-compliance or malabsorption is suspected. Furthermore, based on the response, a treatment plan can be implemented using once or twice weekly dosing which can improve compliance and treatment outcomes.