MR imaging in thyroid disorders: correlation of signal intensity with Graves disease activity.

Thirty-six patients with a variety of thyroid disorders and eight healthy subjects were studied with T1- and T2-weighted magnetic resonance (MR) imaging. Solid benign nodules, malignant tumors, and inflammatory conditions were not distinguishable by thyroidal MR signal intensity, but almost all patients with Graves disease had a moderate to marked diffuse increase in signal intensity at both settings. Quantitative evaluation showed that in these patients, the thyroid-muscle signal intensity contrast ratio was linearly related to both the serum thyroxine (T4) level and the 24-hour radioactive iodine uptake. In three patients treated with iodine 131, this contrast ratio rose or fell in parallel with the serum T4 level and 24-hour radioactive iodine uptake. Either parenchymal changes or increased vascularity in Graves disease, or both, could produce these findings. In patients without Graves disease, signal intensity was not correlated with serum T4 levels. These findings suggest that MR signal intensity may reflect the activity of the stimulatory process in Graves disease and may therefore be a useful measure of thyroid function in this disorder, with both diagnostic and prognostic value.