What does it mean if only T4 is elevated in the “three thyroid function tests”?

Among the three thyroid function tests, an isolated elevation of T4 suggests increased thyroid hormone synthesis and secretion, as well as elevated intrathyroidal thyroid hormone stores. Interpretation requires careful analysis based on individual clinical circumstances:

1. The “three thyroid function tests” refer to T3, T4, and TSH. If only T4 is elevated while both T3 and TSH remain within normal limits, thyroid function is considered normal. Further evaluation with thyroid ultrasound is recommended, along with assessment of the patient’s clinical symptoms. If the thyroid ultrasound is unremarkable and the patient is asymptomatic, no specific treatment is required—only routine follow-up testing is advised.

2. In hyperthyroidism, both T3 and T4 are typically elevated, accompanied by a suppressed TSH level. Therefore, isolated T4 elevation—with normal T3 and TSH—does not support a diagnosis of hyperthyroidism.

When T4 is elevated, it is advisable to reduce intake of iodine-rich foods, such as seaweed, laver (purple laver), and iodized salt.