Introduction: The treatment of subclinical hypothyroidism in elderly persons is controversial. Previous studies have shown that there are interactions between kidney and thyroid function, but data regarding interventions that target thyroid function in elderly patients are scarce. We aimed to investigate the impact of thyroid hormone therapy on the estimated glomerular filtration rate (eGFR) in elderly patients. Material and methods: Ninety elderly patients aged >= 65 years with subclinical hypothyroidism were followed for 36 months in our case-control study. The changes in the eGFR in patients with and without thyroid hormone replacement therapy were compared. The adverse effects during the treatment period were noted. Results: The eGFR of both groups was similar at the beginning of the study (43.4 +/- 6.1 vs. 42.8 +/- 5.9 ml/min/1.73 m(2); p = 0.62). With the decline in thyroid stimulating hormone levels after treatment, the eGFR of the treatment group significantly improved compared with the control group (45.8 +/- 4.8 vs. 35.8 +/- 5.3 ml/min/1.73 m(2); p < 0.001); the eGFR increased rapidly over the first 6 months and then plateaued. No patients withdrew from the study, but the therapeutic dose was decreased in two patients due to angina pectoris. Conclusions: Thyroid hormone therapy preserved renal function in elderly patients. Appropriate individual treatment should be considered in elderly patients with subclinical hypothyroidism.