From the Guidelines
Reverse Triiodothyronine (rT3) should be checked in specific clinical scenarios where thyroid hormone resistance, euthyroid sick syndrome, or a discrepancy between clinical symptoms and standard thyroid test results is suspected, particularly when a patient has normal TSH and T4 levels but continues to experience hypothyroid symptoms. When evaluating the need for rT3 testing, it's essential to consider the patient's overall clinical presentation, including symptoms such as fatigue, weight gain, and dry skin, which may indicate hypothyroidism despite normal TSH levels 1. Some specific clinical scenarios that may warrant rT3 testing include:
- Severe illness
- Malnutrition
- Chronic stress
- Liver disease
- Patients on thyroid medication who aren't responding as expected In these situations, rT3 testing can help identify if the body is converting T4 to inactive rT3 instead of active T3, which can occur during physiological stress as a protective mechanism to conserve energy 1. It's crucial to interpret rT3 levels alongside free T3 and free T4 levels for a complete picture of thyroid hormone metabolism. No specific preparation is needed for the rT3 test, which requires a simple blood draw. However, it's essential to note that the provided evidence does not directly address the use of rT3 testing, but it does emphasize the importance of monitoring thyroid function with TSH measurements and considering clinical symptoms when evaluating thyroid function 1.
From the Research
When to Check Reverse Triiodothyronine (rT3)
- rT3 should be checked in patients with hypothyroidism who are being treated with levothyroxine (LT4) replacement therapy, especially when there are concerns about overtreatment or undertreatment 2.
- In critically ill patients, rT3 and T3/rT3 ratios can be prognostic markers for survival and should be checked on the first day of hospitalization 3, 4.
- rT3 levels can rise to peak levels by day 3 of hospitalization in patients with acute illness, and then return to pre-admission euthyroid levels 5.
- In patients with severe acute illness, rT3 values exceeding 0.61 ng/mL can have prognostic value for mortality 4.
- Annual check-ups for patients on LT4 replacement therapy usually suffice, but dose adjustments may be necessary in certain situations, such as pregnancy or when medications are used that interfere with T4 absorption or metabolism 6.
Clinical Considerations
- The measurement of rT3 can help identify patients who are at risk of developing hyperthyroidism due to overtreatment with LT4 2.
- The rT3:T4 ratio can be used to assess the adequacy of LT4 replacement therapy in patients with hypothyroidism 2.
- Changes in rT3 levels can occur in response to alterations in thyroid hormone metabolism during acute illness, and may not necessarily require changes in LT4 replacement therapy 5, 3.