There are two thyroid hormones, thyroxine (T4) and triiodothyronine (T3), that are secreted by the thyroid gland. Thyroid is used for Thyroid hormone therapy. Thyroid hormone therapy is the use of synthetic thyroid hormones to raise abnormally low levels of thyroid hormones. Often used to treat an underactive thyroid that is secreting little or no thyroid hormones, thyroid hormone therapy is usually administered in tablet form. The most commonly prescribed thyroid hormone replacement is pure synthetic thyroxine (T4).

Doctors often stress that not all tissues that need thyroid hormone are equally able to convert thyroxine to triiodothyronine, the active form of the hormone. Nevertheless, most patients with hypothyroidism (reduced thyroid function) are treated only with thyroxine.

Patients with hypothyroidism show greater improvements in mood and brain function if they receive treatment with two types of thyroid hormones instead of one. Hypothyroidism, where the gland has ceased to function or has been removed, is usually treated with daily doses of thyroxine hormone. But the researchers found that substituting another thyroid hormone, triiodothyronine, for some of the thyroxine dose led to improvements in mood and in neuropsychological functioning.

To determine whether patients would benefit from receiving triiodothyronine as well, 31 women and 2 men who lacked a functioning thyroid and were dependent on thyroxine, were studied. On 6 of 17 measures of mood and cognition — a catchall term that refers to language, learning and memory — the patients scored better after receiving thyroxine plus triiodothyronine than after receiving thyroxine monotherapy.

No score was better after monotherapy than after combination treatment, the study group determined. The doctors also detected biochemical evidence that thyroid hormone action was greater after treatment with thyroxine plus triiodothyronine.