Do thyroid hormone levels change during infection?

An infection can potentially increase thyroid hormone levels, but the relationship between infections and thyroid hormone levels is complex. During illness, including infections, the body's metabolism and hormonal regulation can be significantly affected. This is part of a phenomenon known as "non-thyroidal illness syndrome" or "sick euthyroid syndrome."

In some cases, infections can lead to increased levels of certain thyroid hormones, like free T3, especially in the early phases of the illness. However, as the illness progresses, it's more common to see a decrease in T3 levels. This is because the body reduces the conversion of T4 (thyroxine) to T3, leading to lower T3 levels and an increase in reverse T3, which is an inactive form of the hormone. This adaptation is thought to be a way for the body to conserve energy during illness.

However, it's important to note that changes in thyroid hormone levels during illness don't necessarily indicate thyroid dysfunction. The alterations in hormone levels are typically part of the body's response to the illness. Once the infection or illness resolves, thyroid hormone levels usually return to their normal range.

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