Endocrine Abstracts

Serum calcitonin (CT) screening of thyroid nodules is a highly sensitive test for early diagnosis of medullary thyroid carcinoma (MTC) and is used for initial diagnosis and post-surgical follow-up. MTC is neuroendocrine tumor derived from parafollicular C cells which produce calcitonin. It is a rare tumor, accounting for 5-10% of all thyroid carcinomas, with a favorable prognosis if diagnosed early. A 74-year-old patient was referred to our clinic due to elevated basal CT levels. Routine thyroid ultrasonography revealed hypoechoic nodule in the left lobe 14.5x9.1 mm, with multiple calcifications (TIRADS 4a), and several nodules up to 12 mm in the right lobe. The initial thyroid hormone status showed high CT (203 pg/ml ; ref.<9.52 pg/ml -ECLIA), confirmed by repetition in the same laboratory. During the additional diagnostic at our clinic, basal CT levels were measured by two different assays, and both showed normal findings of basal CT (CT 2.8 ng/l; ref. 1-10.1 ng/l-IRMA); CT<0.5 ng/l (0-9.1 ng/l-ECLIA). Due to suspicion of interference, we performed serial dilutions of the serum that showed linearity (dilutions: 1:1, 1:2, 1:5, 1:10; CT 2.8; 5.8; 10.4; 11.5 ng/l, respectively -IRMA). Also, calcium stimulation test was performed and calcitonin response was determined in both laboratories. Results showed normal response in both laboratories (IRMA: 2.8; 6.8; 10.8; 11.3; 5.0 ng/l; ECLIA: <0.5; 4.0; 5.3; 5.0; 3.2 ng/l). CEA was normal (4.4; 4.3 μg/l). Fine-needle aspiration biopsy of the suspicious nodule was performed, and cytopathology showed atypia of unknown significance (Bethesda classification system III). Due to pathological lymphadenopathy of the left side of the neck and suspicious ultrasound characteristics of the thyroid nodule, the patient was referred to surgery. Total thyroidectomy and left neck dissection were performed. PH diagnosis: multifocal papillary thyroid carcinoma (PTC). Tumor 15x10x1 mm in the left lobe and isthmus with infiltration of the capsule and muscle, and one focus of PTC 3 mm in the same lobe, and 3/9 metastatic LN with extracapsular extension, left, in region III and IV. Control CT measured in the initial laboratory was still elevated postoperatively (151 pg/ml), while normal measured by IRMA (4.5 ng/l). Based on the results, we concluded that the falsely elevated CT observed in the first laboratory was due to immunoassay interference. Immunoassay interference in CT determination is uncommon, but it can affect the course of diagnosis and treatment, so it should be considered in cases of conflicting laboratory results. The right diagnosis may prevent patient from complex diagnostic and therapeutic procedures.