Levothyroxine absorption testing in refractory hypothyroidism following thyroidectomy in an adolescent girl: distinguishing pseudo-malabsorption from true malabsorption

Lifelong levothyroxine replacement therapy is needed for the treatment of post-thyroidectomy-hypothyroidism. Monitoring of thyroid function tests aids in achieving clinical and biochemical euthyroid status which is sometimes difficult to achieve because of factors like poor adherence and gastrointestinal malabsorption. Persistent elevation of thyroid-stimulating hormone after thyroidectomy may reflect nonadherence, drug interactions, or true malabsorption and levothyroxine absorption testing can help distinguish these possibilities. We present the case of an adolescent girl who was referred by the ENT department to pediatric endocrinology clinic for assessment of thyroid function after she had undergone thyroid surgery for papillary carcinoma of the thyroid gland. She was started on treatment with levothyroxine in once-daily dosing. She remained on regular follow-ups and her thyroid function tests were reported within the normal range until almost a year later when she reported persistently high TSH levels prompting an increase in the drug dosage but despite maximum doses, the levels remained the same. Her history was not suggestive of nonadherence to treatment. Hence, we performed the levothyroxine absorption test to rule out any malabsorption of levothyroxine or pseudo-malabsorption due to noncompliance with treatment. The test demonstrated adequate levothyroxine absorption under supervised conditions (fT4 increased from 1.04 ng/dL at baseline to 1.87 ng/dL at 120 min and 2.46 ng/dL at 240 min). The findings, together with subsequent improvement after daily supervised ingestion, strongly supported pseudo-malabsorption related to inconsistent administration; they did not independently prove nonadherence. Levothyroxine absorption testing is a practical tool in adolescents with refractory post-thyroidectomy hypothyroidism. It can demonstrate adequate absorption under supervised conditions and, when interpreted with the clinical context and subsequent response to supervised administration, can guide management while avoiding unnecessary dose escalation.