Four groups of 6 rats were given ad libitum diets with over 60% of the energy as sucrose, fructose or glucose for 10 weeks, and were compared to animals given chow. Assessment of thyroid activity was made using metabolic rate, thyroid 123I uptake, and serum thyroid fraction measurement before, after 5 and 10 weeks on the diet, and 6 weeks after return to a control diet. The major effects were that the thyroid 123I uptake was significantly less than control in the rats after 10 weeks on a high carbohydrate diet. Only the animals on the fructose diet showed any consistent and significant falls in serum T3 and FT3, and T4. These findings suggest that the type of dietary carbohydrate may affect thyroid function in rats.