Background: Hypothyroidism has been associated with impaired urinary concentrating ability. However, previous reports on thyroid hormone and urinary concentrating ability in humans only studied a limited number of patients with autoimmune thyroid disease or used healthy controls instead of paired analysis within the same patients. Objective: To study the urinary concentrating ability in athyreotic patients with differentiated thyroid cancer on and off levothyroxine treatment as they are exposed to different thyroid states as part of their treatment in the absence of an autoimmune disease. Design and Methods: We studied 9 patients (mean age of 42.7 years) during severe hypothyroid state (withdrawal of levothyroxine before radioactive iodine therapy) and TSH-suppressed state (on levothyroxine therapy). At these two points, serum and urine samples were collected after 14 h of overnight fasting without any food or drink. Results: Serum and urine osmolality were not significantly different between on and off levothyroxine treatment. Serum creatinine levels were significantly higher in patients off versus on levothyroxine treatment (87.0 vs. 71.0 µmol/L, respectively; p = 0.044) and, correspondingly, the estimated glomerular filtration rate was significantly lower (89.6 vs. 93.1 mL/min, respectively; p = 0.038). Conclusion: Short-term, severe hypothyroidism has no effect on urinary concentrating ability. Our study confirms the well-known effects of thyroid hormone on serum creatinine concentrations.