Elevated TSH in a Thyroidectomized Patient: Differential Diagnosis Beyond Noncompliance with Treatment

A 42-year-old-woman born in Peru visited our outpatient clinic in 2005 after noticing a single thyroid nodule with progressive enlargement. She denied compressive symptoms and had no relevant personal history. Her father had hyperthyroidism, but there was no family history of thyroid carcinoma. She was clinically euthyroid and presented with a 2.0-cm nodule in the left thyroid lobe. Fine-needle aspiration biopsy of the nodule was consistent with papillary thyroid carcinoma. The patient underwent total thyroidectomy in February 2005. Histopathology confirmed classic variant papillary carcinoma with a size of 1.8 cm, invasion of soft tissues, and vascular embolization (pT3N0Mx). In October 2005, the patient underwent radioiodine ablation treatment with 200 mCi of radioiodine (131I). A posterior body scan detected the tracer only in the anterior cervical region. She exhibited excellent response to treatment, with stimulated thyroglobulin levels <2.0 ng/mL and negativity for antithyroglobulin antibodies. During follow-up, she was treated with substitutive doses of levothyroxine (LT4)3 until 2015, with no evidence of disease seen with cervical ultrasonography. Her thyroid-stimulating hormone (TSH) remained within the normal reference range during this period. In February 2016, the patient was using 1.45 μg/kg/day of LT4, and an in-house assay revealed a TSH of 5.7 mIU/L (a monoclonal antibody-based immunofluorometric assay with sensitivity of 0.05 mIU/L; intraassay …