Serum thyroxine (T4) and other thyroid function tests were studied in 14 patients with subacute thyroiditis and compared with the same parameters in 32 patients with untreated thyrotoxicosis. The mean values of serum T4 and protein-bound iodine (PBI) were found to be elevated to the same extent in the two groups and the calculated T4 iodine did not differ significantly from the PBI in any of the groups. The resin-T3-test and the basal metabolic rate (BMR) mean values were significantly lower in patients with subacute thyroiditis than in patients with thyrotoxicosis. The serum T4 determination based on competitive protein-binding was not influenced by other organic iodinated products, and our results indicate that the elevated serum PBI in subacute thyroiditis is largely due to T4. The lower BMR in patients with subacute thyroiditis is possibly explained by a difference in the thyroxine binding protein (TBP) binding capacity and free T4 in the serum between patients with subacute thyroiditis and those with thyrotoxicosis.