We read the manuscript entitled “Levothyroxine absorption test results in patients with TSH elevation resistant to treatment” with great interest and pleasure [1]. We thought some points should be clarified so we decided to explain the test result discrepancies. İn the article to determine the absorbtion rate in patients with high levels of thyroid-stimulating hormone (TSH) despite receiving adequate doses of levothyroxine;
% L4 absorbtion: [(peak ΔT4 × volume distribution) ÷ administered dose of LT(μg)] × 100
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Volume distribution(dL): 4.42 × body mass index
formula is used and according to the formula, those with an absorbtion result of more than 60–80% are considered normal. Considering the five patients evaluated in the study;
Absorption rates were not consistent with the data in the study Table 1. In addition, very high peakT4 values are required for the 60–80% absorption rate required for the test to be normal. In the study named “The clinical utility of free thyroxine in oral levothyroxine absorption testing”, in which the formula is taken as reference, calculations were made with total T4 and free T4 values and it was observed that similar incompatibility was detected in the same way [2].
Table 1
LT4 absorption test result
Patient | Basal fT4a | Peak fT4 | BMI (kg/m2) | Absorption (%) |
|---|---|---|---|---|
1 | 0.636 | 0.639 | 27 | 6 |
2 | 0.290 | 0.440 | 27 | 40 |
3 | 0.400 | 2.000 | 47 | 166 |
4 | 0.200 | 1.300 | 26 | 97 |
5 | 0.530 | 1.980 | 30 | 90 |
As a result, the reliability of this formula, which is used to evaluate the absorption of levotroxin in patients with persistent elevation of TSH despite levothyroxine replacement therapy, is controversial and there are studies in which an increase of 50–100% in basal free T4 is accepted to evaluate malabsorption [3].
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Compliance with ethical standards
Conflict of interest
The authors declare no competing interests.
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