Levothyroxine Monotherapy Remains the Standard of Care for Hypothyroidism
Levothyroxine (LT4) monotherapy is the established standard treatment for hypothyroidism, and current evidence does not support routine use of combination therapy with liothyronine (T3) for the general hypothyroid population 1, 2, 3. However, a trial of LT4+LT3 combination therapy may be considered in a highly select subset of patients who remain symptomatic despite achieving biochemical euthyroidism on LT4 alone 3, 4.
Evidence Against Routine Combination Therapy
Thirteen clinical practice guidelines from North America, Europe, and South America uniformly conclude that LT4 monotherapy is the standard of care, with insufficient evidence to recommend widespread combination therapy 2.
Only the 2012 European Thyroid Association (ETA) and 2015 British Thyroid Association (BTA) guidelines suggest combination therapy could be used, but solely as an experimental treatment in specific circumstances 2, 4.
Current clinical evidence is not sufficiently strong to support LT4/LT3 combination therapy in patients with hypothyroidism, as results of studies on clinical symptoms and thyroid-responsive genes have not been conclusive 5.
The 5-10% Problem: Persistent Symptoms Despite Normal TSH
Approximately 5-10% of LT4-treated hypothyroid patients with normal serum TSH continue to experience persistent symptoms 4, 6.
Before considering combination therapy, clinicians must systematically exclude other causes of persistent symptoms, including awareness of chronic disease, presence of associated autoimmune diseases, thyroid autoimmunity per se, and non-thyroid conditions 4, 5.
Candidate Selection for Trial Combination Therapy
Combination therapy should only be considered in patients who meet ALL of the following criteria:
Compliant LT4-treated hypothyroid patients with persistent complaints despite serum TSH values within the reference range 4.
Patients who have previously received support to deal with the chronic nature of their disease 4.
Associated autoimmune diseases have been excluded 4.
Non-thyroid causes of symptoms have been aggressively investigated and ruled out 5.
Treatment should only be instituted by accredited internists/endocrinologists 4.
Dosing Protocol for Combination Therapy
When initiating a trial of combination therapy, use the following approach:
Reduce the LT4 dose by 25 mcg/day and add 2.5-7.5 mcg liothyronine (LT3) once or twice daily as an appropriate starting point 3.
Start combination therapy with an LT4/LT3 dose ratio between 13:1 and 20:1 by weight, administering LT4 once daily and dividing the daily LT3 dose into two doses 4, 6.
Currently available combined preparations all have an LT4/LT3 dose ratio of less than 13:1 and are not recommended 4.
Monitoring and Safety Considerations
Close monitoring is indicated, aiming to normalize serum TSH and free T4, while also achieving normal serum free T4/free T3 ratios 4, 6.
Transient episodes of hypertriiodothyroninemia with these doses of LT4 and LT3 are unlikely to exceed the reference range and have not been associated with adverse drug reactions 3.
Treatment should be discontinued if no improvement is experienced after 3 months 4.
Long-Term Safety Data
Trials following almost 1000 patients for almost 1 year indicate that similar to LT4, therapy with LT4+LT3 can restore euthyroidism while maintaining a normal serum TSH 3.
An observational study of 400 patients with a mean follow-up of approximately 9 years did not indicate increased mortality or morbidity risk due to cardiovascular disease, atrial fibrillation, or fractures after adjusting for age when compared with patients taking only LT4 3.
Role of Genetic Polymorphisms
Polymorphisms in deiodinase genes (particularly D2) that encode the enzymes converting T4 to T3 in the periphery may provide potential mechanisms underlying unsatisfactory treatment results with LT4 monotherapy 5, 6.
Patients with documented polymorphism of the deiodinase 2 (D2) genes could potentially benefit from the addition of LT3 to LT4 6.
Desiccated Thyroid Extract (DTE)
Desiccated thyroid extract is a form of combination therapy with an LT4/LT3 ratio of approximately 4:1; the mean daily dose needed to normalize serum TSH contains approximately 11 mcg T3, though some patients may require higher doses 3.
DTE remains outside formal FDA oversight, and consistency of T4 and T3 contents is monitored by the manufacturers only 3.
Critical Pitfalls to Avoid
Do not use combination therapy as first-line treatment for newly diagnosed hypothyroid patients—they should be treated with LT4 3.
Do not initiate combination therapy without first systematically excluding non-thyroid causes of persistent symptoms 4, 5.
Do not continue combination therapy beyond 3 months if no improvement is documented 4.
Avoid using commercially available combined preparations with LT4/LT3 ratios less than 13:1 4.
Do not prescribe combination therapy outside of specialist endocrinology care 4.
Treatment Algorithm
For newly diagnosed hypothyroidism:
- Start LT4 monotherapy and titrate to normalize TSH 3.
For patients with persistent symptoms on LT4:
- Confirm TSH is within reference range and LT4 dose is appropriate 4.
- Provide psychological support for chronic disease management 4.
- Systematically exclude associated autoimmune diseases 4.
- Aggressively investigate non-thyroid causes of symptoms 5.
- If all above steps completed and symptoms persist, refer to endocrinologist 4.
- Consider trial of LT4+LT3 (reduce LT4 by 25 mcg, add 2.5-7.5 mcg LT3 divided twice daily) 3.
- Monitor closely for 3 months 4.
- Discontinue if no benefit; continue if clear improvement 4.
L-T4 + L-T3 combination therapy should be considered solely as an experimental treatment modality 4.