This clinical case is published for the first time. Case reports are relevant for generalizing medical information by pooling clinical cases to identify phenotypic features of inherited diseases, thereby avoiding future medical errors. Sipple syndrome is a rare genetic disorder affecting the thyroid, parathyroid, and adrenal glands caused by mutations in the RET gene. Tumors can be asymptomatic making correct diagnosis difficult. Clinical Case: Patient S., 38, a working resident of Vologda, has been under observation for multinodular goiter since childhood. In 2013, a thyroid ultrasound revealed small nodules with calcifications in the right lobe and a conglomerate nodule in the left lobe. In 2013, a thyroidectomy was performed (molecular genetic testing was not performed). In vivo histological examination revealed a multinodular goiter with amyloidosis and a multicentric growth pattern in the thyroid gland. The patient has been followed at this local city clinic since 2015. In 2023, the patient underwent surgery for a parathyroid adenoma. In November, DNA fragments were sequenced, confirming the cause of the disease at the molecular genetic level. Results: A heterozygous variant (rs75996173) was detected in the RET gene (NM 020975.6) in exon 11, c.1901G>A, p.C634Y.

A medical history was collected from the patient's own testimony. The diagnosis was established based on anamnestic data, biochemical studies, and instrumental imaging, genotyping.

The patient has a daughter. In 2017, during a routine pre-school medical examination, a thyroid ultrasound was performed, which revealed a 3.7 mm thyroid nodule. In 2021, at age 11, the daughter was hospitalized with a persistent right lobe nodule (EU-TIRADS2). Hormonal profile revealed euthyroidism, and calcitonin levels were elevated to 24.5 pg/mL. The daughter's molecular genetic test results were consistent with those of her mother. Given the genetically confirmed diagnosis of MEN2A, the daughter underwent a prophylactic thyroidectomy in 2021.

The case confirms literature data indicating that MTC is the first MEN2A pathology to develop in childhood. Given the elevated calcitonin levels and a positive family history (with MTC in the mother), multiple endocrine neoplasia type 2 could have been suspected earlier. The mother's elevated calcitonin levels persist despite the absence of signs of relapse, which may be a phenotypic feature of the patient's disease course. Genotyping is not cost-effective, and large-scale studies of any rare disease are virtually impossible.