Levothyroxine (LT4) Absorption Test in a Pregnant Patient: Safety and Efficacy

The safety and efficacy of levothyroxine (LT4) absorption test in pregnant women is yet to be studied. We present a case of a 19-year-old female at 14 weeks of pregnancy who was subjected to LT4 absorption test.

Case Description :

An 18-year-old female with self-reported congenital hypothyroidism was seen in the clinic for elevated TSH at 80uIU/mL and FT4 of 0.29ng/dL. The patient was instructed the appropriate method of LT4 intake and dose-adjusted, but was lost to follow up after a two-week visit without repeat labs. Later at age 19, patient was re-referred for hypothyroidism in pregnancy and seen at 14 weeks with markedly elevated TSH at 109uIU/mL and FT4 at 0.4ng/dL despite daily LT4 therapy at 150mcg. The patient had no history of gastrointestinal disorders such as celiac disease or jejunoileal bypass. She was not on any PPI but was taking LT4 at bedtime with an anti-allergy pill.

Given the risk of uncontrolled hypothyroidism, the patient was extensively counseled of the risk to the fetus and appropriate method of LT4 intake. In about one week, the patient underwent LT4 absorption test to rule out malabsorption. The test was performed in the clinic with the supervised intake of a week’s dose of LT4 calculated based on weight plus estimated pregnancy increment, a total of 1225 mcg of LT4. Her baseline labs on the test day were TSH 92uIU/mL and FT4 0.8ng/dL, suggesting recent compliance during pregnancy. The patient did not take her daily dose of 175 mcg on the day of the loading dose. LT4 absorption test results were consistent with pseudomalabsorption as the patient achieved a 37.5% rise in her FT4 levels (1.1ng/dL) after loading dose in 80min, 75% rise in 140 min (1.4ng/dL) and 162.5% rise in 220min (2.1ng/dL). We did not measure serial TSH levels as it can take up to 4 months of treatment for TSH to normalize. Follow-up labs at one week were not completed due to the patient’s failure to follow up. The patient tolerated the loading dose without any side effects. Patient-reported significant improvement /reversal of her hypothyroid symptoms within 48 hours of administration of the loading dose, including fatigue, joint and muscle pains, constipation, dry skin and insomnia. The patient resumed her LT4 a week after the loading dose, every morning one hour before breakfast with her pre-natal/anti-allergy at night.

Discussion :

Hypothyroidism is a common disorder that has been successfully treated with hormone replacement therapy. However, some cases of hypothyroidism are refractory to treatment. Levothyroxine absorption test helps in differentiating malabsorption from pseudomalabsorption in such patients. LT4 requirements increase with pregnancy as early as the 5th week and stabilize around 21 weeks of gestation. Low concentrations of maternal thyroid hormones during early gestation are associated with fetal neurodevelopmental delays. Therefore, it is essential to achieve a euthyroid state as quickly as possible, making refractory hypothyroidism a more significant challenge in pregnant patients. We report a case of pseudomalabsorption in pregnancy and that LT4 absorption test can be administered safely in pregnancy to diagnose pseudomalabsorption related hypothyroidism.