Because T4 is converted into another thyroid hormone called T3 (triiodothyronine), free T4 is the more important hormone to measure. Any changes show up in T4 first. T3 and T4 help to control how your body stores and uses energy to do its work (metabolism).
A higher than normal level of T4 may be due to conditions that involve an overactive thyroid, including:
- Graves disease.
- Taking too much thyroid hormone medicine.
- Thyroiditis.
- Toxic goiter or toxic thyroid nodules.
- Some tumors of the testes or ovaries (rare)
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