Levothyroxine treatment alone may not resolve symptoms of hypothyroidism and the solution may be adding T3 medication to the treatment. Many people who continue to experience symptoms of low thyroid even though their blood tests have become normal while on T4 medication alone. One study looked at 397 hypothyroid individuals who were treated with T4 only and compared them to 397 people with normal thyroid function. The hypothyroid individuals reported more general health symptoms as well as thyroid-specific symptoms than those with normal thyroid function. Another study showed that cognitive testing scores and the overall well-being of hypothyroid individuals on T4 only were worse compared to those of the general population. Research also demonstrated reduced overall health, poor psychological function, memory impairment, and reduced learning abilities in people on T4 alone, despite normal blood tests. A study showed that rats with removed thyroid glands only regained normal thyroid levels in their organs and tissues with a combination of T4 and T3 supplementation. One theory as to why symptoms can persist despite having normal thyroid blood tests is that some people may be impaired at converting T4 to T3 in certain tissues.
At the tissue level, T3 deficit may be to blame for residual symptoms, perhaps in addition to the deficiency of T3 produced by the thyroid gland itself. In a study where people were given thyroid treatment consisting of both T4 and T3, a significant improvement in mood, cognition, and physical symptoms was reported. These people indicated that they preferred the combination treatment versus T4 treatment only. Another study showed that the use of T3 promotes weight loss and improvements in cholesterol levels.