Changes in thyroid function in nonthyroid illness.

Nonthyroid illness can cause changes in thyroid function that have been described as low triiodothyronine (T(3)) and low thyroxine (T(4)) states. Reduced peripheral conversion of T4 to T(3) explains the low serum T(3) concentration. The explanation for the low serum T(4) level is multifactorial; whether free-thyroxine (FT(4)) level is normal or reduced remains controversial. Cytokines such as tumor necrosis factor alpha, which are produced by the immune system during severe illness, may inhibit thyroid function directly and be responsible for the changes in pituitary-thyroid function.