The serum triiodothyronine (T3) level should be measured next. This patient exhibits signs and symptoms of hyperthyroidism. Even though the available laboratory data are consistent with subclinical hyperthyroidism, the diagnostic evaluation is not complete until the T3 level is checked. Although rare, an elevated T3 level and a normal serum free thyroxine (T4) level may be present in patients with hyperthyroidism. Measurement of the T3 concentration should therefore be obtained in all patients suspected of having thyrotoxicosis. Measurement of the T3 level is not indicated in patients with hypothyroidism because the T3 concentration is conserved and may remain within the normal range, even in patients with significant hypothyroidism.
Measuring the thyroid peroxidase (TPO) antibody titer will not provide additional information about this patient. Determining TPO antibody status is helpful in patients with a mildly elevated serum thyroid-stimulating hormone (TSH) level and is associated with Hashimoto thyroiditis and future risk of developing permanent hypothyroidism. However, this patient demonstrates signs and symptoms consistent with hyperthyroidism.
Repeating the thyroid function tests (TFTs) in 6 weeks may be appropriate if the total or free T3 level is found to be normal. If the T3 is normal, the patient has subclinical hyperthyroidism, and repeating the TFTs may be indicated to determine if the abnormality is transient or permanent.
Ultrasound of the neck is appropriate for the evaluation of this patient if a nodule is suspected. However, this patient's diffusely enlarged thyroid gland is not suggestive of nodular disease. In addition, even if the physical examination were suggestive of nodular disease, the first step would be evaluation of the functional thyroid status.