A cross-sectional study was performed in 19 patients on haemodialysis and in 11 on continuous ambulatory peritoneal dialysis (CAPD) in order to investigate the relationship between thyroid hormones and nutritional status. T4, T3 resin uptake, T3, rT3 and TSH were measured by radio-immunoassay and compared with controls. Nutritional status was assessed by measurements of blood proteins, albumin, γ-globulin, transferrin, arm muscle circumference and triceps skinfold thickness. In haemodialysis, T4, FTI, T3 and rT3 were significantly decreased. TSH was normal. In CAPD, thyroid hormones were normal. In both groups, proteins, albumin, γ-globulin and transferrín were normal. Triceps skinfold thickness was normal in males and females, whereas arm muscle area was dramatically reduced in males and normal in females. In haemodialysis, a negative correlation was found between T3 and proteins (p < 0.01), rT3 versus proteins (p < 0.01) and versus γ-globulin (p < 0.01). In CAPD, a positive correlation was found between T3 and triceps skinfold thickness (p < 0.05). We suggest that the overload of carbohydrate might normalize the thyroid hormones in patients on CAPD. The relationship between thyroid hormones and nutritional status in patients treated by dialysis suggests a putative protective effect of low T3 levels against protein breakdown.