From the FDA Drug Label
When the total serum T4 is low but TSH is normal, a test specific to assess unbound (free) T4 levels is warranted However, the label does not specifically address low T3 with normal TSH. The FDA drug label does not answer the question.
From the Research
Low T3 with normal TSH typically indicates a condition called euthyroid sick syndrome or non-thyroidal illness syndrome, and no specific thyroid treatment is usually needed as the T3 levels typically normalize once the underlying condition improves. This pattern occurs when the body decreases conversion of T4 to T3 during illness, stress, malnutrition, or other systemic problems, while maintaining normal TSH levels. It's generally not a primary thyroid disorder but rather the body's adaptive response to conserve energy during illness. Instead, focus should be on identifying and treating the underlying cause, which might include severe illness, malnutrition, chronic kidney disease, liver disease, major surgery, or severe trauma.
Key Considerations
- Laboratory monitoring of thyroid function during recovery is recommended, but thyroid hormone supplementation is rarely indicated unless there are other signs of true hypothyroidism 1.
- This pattern differs from primary hypothyroidism, where low T3 would typically be accompanied by elevated TSH levels.
- If low T3 with normal TSH persists after recovery from illness, further evaluation may be warranted to rule out other thyroid disorders.
Recent Evidence
- A recent study published in 2025 found that LT4/LT3 combination therapy appears to prevent changes in weight and cholesterol associated with LT4 replacement alone in patients with post-surgical hypothyroidism 2.
- However, the use of liothyronine (T3) in hypothyroidism remains controversial, and current guidelines recommend that hypothyroid patients should be treated with levothyroxine, with combination therapy considered only in select cases 1.
Clinical Implications
- The decision to start treatment with liothyronine should be a shared decision between patient and clinician, taking into account the individual patient's symptoms, medical history, and laboratory results 1.
- Clinicians should not feel obliged to start liothyronine or to continue liothyronine medication provided by other health care practitioners or accessed without medical advice, if they judge this not to be in the patient's best interest 1.