Decreased triiodothyronine levels in the serum usually indicate hypothyroidism or chronic or subacute thyroiditis. Any serious illness can decrease triiodothyronine levels. Abnormally low triiodothyronine may cause symptoms of hypothyroidism including weakness and fatigue, cold intolerance, shortness of breath, weight gain, constipation, cognitive problems, dry skin, hoarseness, and swelling (edema).

Some specific causes of low triiodothyronine are:

• Hypothyroidism
• Chronic thyroiditis
• Subacute thyroiditis
• Nephrotic syndrome
• Drugs that decrease TBG (e.g. anabolic steroids, glucocorticoids)

Elevated triiodothyronine levels may indicate hyperthyroidism, thyroid hormone resistance syndrome, or triiodothyronine toxicosis. Serum triiodothyronine levels can be expected to increase during normal pregnancy, but other tests may be necessary to rule out hyperthyroidism. Elevated triiodothyronine may cause symptoms of hyperthyroidism including excessive appetite, anxiety, heart palpitations, shortness of breath, sweating, weight loss, emotional disturbances, and intolerance to heat. 

Some specific causes of high triiodothyronine are:

• Hyperthyroidism
• Euthyroid hyperthyroxinemia
• Triiodothyronine toxicosis
• Acute thyroiditis
• Pregnancy
• Familial dysalbuminemic hyperthyroxinemia
• Drugs that increase TBG (e.g. estrogens, tamoxifen, opioids)
• Synthetic triiodothyronine treatment