Why does thyroxene fail to replace natural sources of T3 & T4?

The short version: levothyroxine (T4 alone) fails to replace natural thyroid function because the body needs T3, and many people cannot convert enough T4 into T3.

The standard treatment for hypothyroidism is levothyroxine, which is only T4. A healthy thyroid gland produces both T4 and T3 directly. The body has to convert T4 into the active hormone T3 using enzymes called deiodinases. This conversion can fail in a significant number of people. A 2012 review in the European Thyroid Journal [2] explains that a subset of patients on T4 alone have persistent symptoms and low T3 levels despite a normal TSH. The review points to a genetic variant in the DIO2 gene (the Thr92Ala polymorphism), found in about 15% of the population, which impairs the conversion of T4 to T3 in tissues like the brain. This means serum T3 levels do not reflect what is happening inside cells, and the brain can be functionally hypothyroid even when blood tests look normal.

A 2025 case series in the Journal of the Endocrine Society [1] directly demonstrates the problem. Three patients on T4 alone had persistent fatigue and low T3. When their doctors reduced the T4 dose and added liothyronine (synthetic T3), their T3 levels rose and their symptoms resolved. The evidence here is thin—only three patients—but it matches the mechanism described in the larger review. The 2012 review also notes that a meta-analysis of 11 randomized trials found that adding T3 to T4 raised total T3 levels but did not show a clear benefit on symptoms across the whole group [2]. That suggests the benefit is real but limited to the subset of people who cannot convert T4 efficiently.

The bottom line is that T4-only therapy assumes everyone can convert T4 to T3 equally well. The evidence shows that is not true. A significant minority have a genetic or metabolic block in that conversion, and for them, T4 alone is an incomplete replacement. The system does not routinely test for this or offer combination therapy as a first-line option.

My call: for people with persistent symptoms and low T3 despite normal TSH on T4 alone, adding T3 is supported by mechanism and limited clinical evidence. Confidence: moderate, because the best evidence (the meta-analysis) shows no group-wide benefit, and the positive case series is very small.