From the FDA Drug Label
Although liothyronine sodium has a rapid cutoff, its metabolic effects persist for a few days following discontinuance. ... However, the wide swings in serum T3 levels that follow its administration and the possibility of more pronounced cardiovascular side effects tend to counterbalance the stated advantages Liothyronine Sodium Tablets, USP may be used in preference to levothyroxine (T4) during radioisotope scanning procedures, since induction of hypothyroidism in those cases is more abrupt and can be of shorter duration. It may also be preferred when impairment of peripheral conversion of T4 to T3 is suspected.
The decision to combine T3 and T4 for hypothyroidism treatment should be made on a case-by-case basis, considering the individual patient's response and laboratory findings.
- Key considerations include the patient's condition, such as myxedema or congenital hypothyroidism, and the potential for cardiovascular side effects.
- Combination therapy may be preferred in certain situations, such as during radioisotope scanning procedures or when there is suspected impairment of peripheral conversion of T4 to T3.
- Monitoring of serum TSH and free-T4 levels is crucial to assess the adequacy of therapy and adjust the dosage as needed 1.
From the Research
Combining T3 and T4 therapy for hypothyroidism should be considered when patients continue to experience persistent symptoms despite achieving normal TSH levels on T4 monotherapy, as suggested by the most recent study 2. This approach is typically reserved for patients who have tried standard levothyroxine treatment at adequate doses for sufficient time but still report fatigue, depression, cognitive issues, or other hypothyroid symptoms. Some key points to consider when combining T3 and T4 therapy include:
- Common combination regimens include adding 5-25 mcg of liothyronine daily to a reduced dose of levothyroxine, or using desiccated thyroid extract (containing both hormones) 3.
- When initiating combination therapy, the levothyroxine dose should generally be reduced by about 20-30% to avoid overtreatment 4.
- Regular monitoring of thyroid function tests is essential, aiming to maintain TSH within normal range while watching for signs of excess thyroid hormone 5.
- The biological rationale for combination therapy stems from the fact that while most T3 in the body is normally converted from T4, some individuals may have genetic variations affecting this conversion process 6.
- Some patients may simply feel better with both hormones, possibly because combination therapy more closely mimics natural thyroid physiology. However, this approach remains somewhat controversial, as benefits in clinical trials have been inconsistent, and more well-designed randomized clinical trials in selected patients are key in order to make progress 2.