Thyroid function tests.

atrial fibrillation or osteoporosis and makes the illogical extrapolation that a patient given a dose of thyroxine that suppresses or brings the serum thyroxine concentration to higher than the reference range is receiving too much replacement. It is excusable for doctors to believe that if the serum thyroxine concentration is higher than the so-called normal range that the dose of thyroxine should be decreased. In fact, the thyroxine is replacing the thyroxine and tri-iodothyronine, which is produced by a normal gland. The only function of doing a thyroxine estimation is to confirm that the patient is taking the thyroxine and that it is being absorbed. Serum thyroxine concentration, if raised, confirms that the thyroxine replacement dose is inadequate but a suppressed concentration does not necessarily show that the dose is too high. Serum T3 must be kept within the normal range. Before the days of hormone assays, hypothyroid patients received about double the average dose of thyroxine given today, but did not develop osteoporosis or atrial fibrillation. Doses should be judged clinically rather than be governed by misinterpreted hormone results.