In this study free thyroxine (fT4) and free tri-iodothyronine (fT3) levels were assayed in 87 patients with a clinical diagnosis of hypothyroidism who had elevated thyrotrophin (TSH) levels (greater than 5.5 mU/l). fT4 levels were decreased in 72 of these 87 patients. However, only 44 of the 72 patients with low fT4 levels and increased TSH levels had a decreased concentration of fT3. The remaining 28 patients had normal fT3 levels. It appears that fT3 levels have little, if any, place in screening for hypothyroidism.