It is well established that in normal subjects 15% to 20% of the circulating T3 is directly secreted from the thyroid. In hypothyroid patients, it is known that T3 concentrations can be lower when compared to normal people. One could argue that with a combined thyroxine and slow-release T3 treatment, patients could be monitored with T3 measurement. There are two problems with this argument: 1) normal fluctuations of serum T3 varies due to many factors and 2) no evidence showing that a combination T3-T4 therapy makes patients feel better compared to straight T4 replacement for the selected symptomatic patient who feels hypothyroid inspite of normal levels. However, a trial of T3-T4 combination therpay is sometimes attempted.