Introduction: Riedel's thyroiditis (RT) is a rare disease characterized by destructive inflammatory fibrosis of the thyroid gland. It is often part of a systemic disease, idiopathic multifocal fibrosclerosis, which can affect other sites such as the retroperitoneum. We report two cases of RT revealed by retroperitoneal fibrosis (RPF), emphasizing the contribution of biological investigations to the diagnosis and management. Case Reports: Case 1: A 36-year-old female, with no significant medical history, presented with bilateral renal colic and a hard goiter. Laboratory findings revealed renal insufficiency and a normal thyroid panel. Imaging (ultrasound, CT urogram) confirmed RPF. Lymph node and thyroid biopsies confirmed the fibro-inflammatory process. Case 2: A 72-year-old female, with a history of hypertension and diabetes, presented with a long-standing stony goiter. A routine blood test revealed renal insufficiency. Imaging highlighted RPF. Histology after total thyroidectomy confirmed RT. Discussion: Biological investigations play a crucial role at several levels: 1) Screening for systemic involvement (renal failure), 2) Assessment of thyroid status (often normal initially), 3) Search for inflammatory evidence (biological inflammatory syndrome, often present), and 4) Therapeutic monitoring under corticosteroids. The absence of specific autoantibodies makes histology essential for diagnosis. Conclusion: RT is a rare cause of RPF. Although non-specific, biological workup is a key element for diagnosing systemic extension and for follow-up. It must systematically include renal, thyroid, and inflammatory assessments in any suspected case of RT.