PSAT364 High-Dose Levothyroxine to Exclude Malabsorption as a Cause of Refractory Hypothyroidism in Hospitalized Patients: Two Cases

INTRODUCTION: Levothyroxine malabsorption is one of the causes of refractory hypothyroidism. Administration of a single large dose of levothyroxine can help distinguish this from refractory hypothyroidism due to other causes, but little is known about the utility of this test in patients hospitalized for severe hypothyroidism. CASE 1: A 42-year-old woman with Hashimoto's thyroiditis presented with symptomatic hypothyroidism. She reported being compliant with 150 mcg of levothyroxine daily. However, in the past year, her free T4 remained < 0.10 ng/dL on several occasions with TSH > 80 uIU/mL. On admission, free T4 was 0.58 ng/dL, total T4 was 4.9 ug/dL, and TSH was 57.06 uIU/mL. We administered 1000 mcg levothyroxine and found an increase in total T4 at 4 hours (12.1 ug/dL from 4.9 ug/dL). Absorption was found to be 100% at 4 hours implying that she was not taking her levothyroxine appropriately. She was counseled to take levothyroxine at 150 mcg on an empty stomach. At outpatient follow up in 3 weeks, free T4 improved to 1.44 ng/dL and TSH decreased to 1.050 uIU/mL. CASE 2: A 42-year-old woman with hypothyroidism, alcohol hepatitis, chronic pancreatitis and previous Roux-en-Y gastric bypass was admitted with progressive lower extremity edema and shortness of breath. She reported taking levothyroxine 175 mcg each morning with her other medications that included omeprazole and magnesium oxide, along with coffee and milk soon after. Her admission labs showed free T4 0.61 ng/dL, total T4 2.8 ug/dL and TSH 84.4 uIU/mL. After 1000 mcg of levothyroxine her total T4 increased from 2.6 ug/dL to 7.8 ug/dL in 4 hours, indicating 100% absorption. The reason for her inadequate outpatient response was probably the combined use of omeprazole, magnesium oxide and coffee with milk, being taken with levothyroxine. She was asked to continue levothyroxine 150 mcg on an empty stomach. Method: Patients were made NPO at midnight prior to the morning of the study. Baseline total T4 was obtained at 6: 00 AM and 1000 mcg of oral Levothyroxine was administered while the patient was monitored on telemetry. Total T4 was measured at 2 and 4 hours after administration. Percentage absorption (% absorbed) = [[Increment TT4 (mcg/dL)×10 (dL/L)]/[total administered LT4 (mcg)]]×Vd (L)X100; Increment TT4 = [Peak TT4] - [Baseline TT4]; Vd (volume of distribution) = 0.442×BMI(1) CONCLUSION: These cases indicate that a 1000 mcg T4 absorption study is safe and reasonable to perform in patients who are hospitalized for profound hypothyroidism when outpatient evaluation has been ineffective or challenging. 1 Gonzales KM, Stan MN, Morris JC 3rd, Bernet V, Castro MR. The Levothyroxine Absorption Test: A Four-Year Experience (2015-2018) at The Mayo Clinic. Thyroid. 2019 Dec;29(12): 1734-1742. Presentation: Saturday, June 11, 2022 1:00 p.m. - 3:00 p.m.