What happens to free T3 levels in primary hyperthyroidism?

In primary hyperthyroidism, the levels of free T3 (triiodothyronine) typically increase drastically. This condition is characterized by the overproduction of thyroid hormones due to issues within the thyroid gland itself, such as in Graves' disease or toxic nodular goiter. As a result, the thyroid gland releases excess amounts of both T3 and T4.

Since T3 is the more active form of thyroid hormone compared to T4, its levels rise significantly in response to the increased production by the thyroid gland. The elevation of free T3 is a hallmark of primary hyperthyroidism and can contribute to the symptoms experienced by patients, such as weight loss, heat intolerance, and increased heart rate.

Monitoring free T3 levels helps in the diagnosis and management of hyperthyroidism, as changes in these levels can indicate how well the condition is being controlled with treatment or the severity of the disease itself. In contrast, during states of euthyroidism or hypothyroidism, the free T3 levels would be expected to be normal or low, respectively, which highlights the distinct nature of primary hyperthyroidism.