From the Guidelines
Elevated Triiodothyronine (T3) levels are a significant indicator of hyperthyroidism, requiring prompt evaluation and management to prevent serious complications, as outlined in the 2021 ASCO guideline update 1. The clinical significance of high T3 levels lies in their association with hyperthyroidism, a condition that can cause a range of symptoms including rapid heart rate, weight loss, anxiety, tremors, heat intolerance, and sleep disturbances.
- The most common cause of elevated T3 is Graves' disease, an autoimmune condition, though other causes include toxic multinodular goiter, thyroiditis, excessive thyroid medication, and certain pituitary disorders.
- Diagnosis usually requires additional testing including TSH (typically suppressed in hyperthyroidism), Free T4, and possibly thyroid antibodies, as recommended in the management of immune-related adverse events in patients treated with immune checkpoint inhibitor therapy 1.
- Treatment depends on the underlying cause and severity of symptoms, with options including anti-thyroid medications, beta-blockers, radioactive iodine therapy, or thyroid surgery, and should be guided by the grading of symptoms, with consideration of endocrine consultation for persistent thyrotoxicosis or severe symptoms 1.
- Elevated T3 can lead to serious complications if untreated, including heart problems, osteoporosis, and in severe cases, thyroid storm—a life-threatening condition requiring immediate medical attention, highlighting the importance of prompt evaluation and management.
- According to the 2021 ASCO guideline update, patients with severe symptoms (G3-4) should have their immune checkpoint inhibitor therapy held until symptoms resolve to baseline with appropriate therapy, and should be considered for hospitalization and endocrine consultation 1.
From the Research
Significance of Elevated Triiodothyronine (T3) Levels
Elevated T3 levels can be an indicator of hyperthyroidism, a condition where the thyroid gland produces excess thyroid hormones 2. Hyperthyroidism can lead to various health issues, including:
- Cardiac arrhythmias
- Heart failure
- Osteoporosis
- Adverse pregnancy outcomes
- Unintentional weight loss
- Increased mortality
Causes of Elevated T3 Levels
The most common cause of hyperthyroidism is Graves' disease, which has a global prevalence of 2% in women and 0.5% in men 2. Other causes of hyperthyroidism and thyrotoxicosis include:
- Toxic nodules
- Thyrotoxic phase of thyroiditis
Diagnosis and Treatment
Elevated T3 levels can be diagnosed through thyroid function tests, including TSH, free T3, and free T4 measurements 2, 3. Treatment options for hyperthyroidism include:
- Antithyroid drugs
- Radioactive iodine ablation
- Surgery
- Thyroid hormone replacement therapy for hypothyroidism 4, 5
T3 Testing in Hypothyroidism
T3 testing may not be necessary in the assessment of levothyroxine (T4) over-replacement in hypothyroid patients, as T3 levels can be normal in over-replaced patients 3. However, T3 levels can be useful in differentiating between Graves' disease and destruction-induced thyrotoxicosis 6.
Key Findings
- Elevated T3 levels can indicate hyperthyroidism and increase the risk of health complications 2.
- T3 testing may not be necessary in hypothyroid patients on levothyroxine replacement therapy 3.
- The T3 to T4 ratio can be a helpful index in differentiating between Graves' disease and destruction-induced thyrotoxicosis 6.