Apparent isolated elevation of serum triiodothyronine level in a patient with a thyroid nodule

Cystic thyroid nodules most commonly occur with a euthyroid state and appear as cold lesions on radioactive iodine scan. I describe a 32-year-old woman who presented during the third trimester of her second pregnancy with a toxic cystic lesion of the thyroid. In studies performed post partum, the serum free thyroxine (T4) index was normal, while the triiodothyronine (T3) level was elevated and thyrotropin level was low and unresponsive to thyrotropin-releasing hormone stimulation. Iodine 123 thyroid scan demonstrated localized nonsuppressible uptake over the nodule. Multiple echo-free (fluid-filled) cavities were observed by ultrasound study. Thyroid lobectomy demonstrated a thyroid colloid nodule with areas of cystic degeneration containing hyperplastic papillary projections. Postoperatively, the patient developed transient hypothyroidism followed by spontaneous recovery to euthyroidism with normal function of the suppressed gland. Blood obtained from the venous effluent of the thyroid nodule had extremely high T3 and T4 concentrations: 359.1 nmol/L and 2420 nmol/L, respectively, while peripheral venous blood showed isolated elevation of T3, to 4.6 nmol/L (normal, 1.2 to 3.1 nmol/L), and a normal T4 level of 116 nmol/L (normal, 64 to 154 nmol/L). Serum T3 and T4 thyroidal/peripheral ratios were close to one in two euthyroid patients undergoing neck surgery. Thus, thyroid nodules may present with isolated elevation of serum T3 levels despite increased thyroidal release of T4.