Treatment With Thyroid Hormone

To the Editor.—  In reference to the editorial by Dr Cooper,¹ practitioners who continue to use thyroid extract despite criticism from their colleagues do so for a good reason: it sometimes works better than other thyroid preparations. I frequently see patients receiving long-term levothyroxine sodium therapy who, despite supposedly appropriate treatment, still complain of fatigue, depression, cold extremities, dry skin, or other symptoms. After changing to an equivalent (or sometimes slightly less than equivalent) dosage of desiccated thyroid, these symptoms improve considerably.One might speculate that the therapeutic advantage of thyroid extract is due to diiodotyrosine. As much as 38% of the iodine present in the thyroid gland is in the form of this compound, and diiodotyrosine is found naturally in the blood. In one study,² diiodotyrosine was effective in the treatment of thyrotoxicosis, suggesting that this compound may somehow modulate the effect of thyroid hormones.We should