Figure 4.

The protocol of the LT4 absorption test and the simulated alteration of serum total T4 levels. Patient medical history should be taken and any possible conditions responsible for impaired LT4 bioavailability should be ruled out prior to the test. After fasting overnight, a high dose of LT4 (in tablet form for most patients) is ingested with water by the patient. The standardized dose is determined by the patient's age and body mass index (BMI), namely, 1000 µg for patients aged between 18 and 65 with a BMI <40 kg/m², 1500 µg for patients aged between 18 and 65 and a BMI ≥40 kg/m², and 600 µg for patients aged >65. Blood specimens are collected for 6 hours (at 0, 1, 2, 3, 4, and 6 hours, respectively) and are subsequently tested for total T4 and TSH. At the end of the test, vital signs and symptoms are reassessed. In the process, no food or medication is permitted to be ingested. The term of LT4 absorption test can be extended to 72 hours postdosing (without fasting) or be shortened to 2 hours. The percentage of LT4 absorption is calculated with the formula below. A percentage of 60% is defined as normal absorption which indicates a diagnosis of pseudomalabsorption. % Absorbed = [Increment TT4 (µg/dL) × 10/total administered LT4 (µg)] × Vd (L) × 100 Increment TT4 = peak [TT4] – baseline [TT4] Vd (volume of distribution) = 0.442 × BMI. Abbreviations: Vd, volume of distribution. The figure was partly generated using illustrative elements from Servier Medical Art, provided by Servier, licensed under a Creative Commons Attribution 3.0 unported license.