Thyroid uptake of Tc-99m pertechnetate and its relationship with TSH levels in euthyroid and hyperthyroid patients

Full text: Introduction: In an earlier small study, we found that the rate of uptake of Tc-99m pertechnetate by the thyroid could be used to differentiate between euthyroid and hyperthyroid patients. The objective of this study was to expand on this earlier study to a larger patient group and to include two other thyroid uptake parameters that might prove useful in thyroid investigations, i.e., thyroid/salivary gland uptake and the thyroid Patlak constant. Materials and methods: Only those patients whose thyroid function tests showed them to have appeared to have intact hypothalamic and pituitary function were selected for the study. Software was written in VPL to create time/activity curves for thyroid and salivary glands for a three minute dynamic acquisition, immediately post injection of Tc-99m. The software then calculated the thyroid parameters that were plotted using MS Excel. Results: Scattergrams of thyroid uptake, thyroid/salivary gland uptake and Patlak constant for 110 patients were plotted against serum TSH levels. Each scattergram showed a set-point similar to the set-point seen in the well-known free T4 vs TSH scattergram. In other words, when thyroid uptake, thyroid/salivary gland uptake or Patlak constant rose above a certain level, the value of TSH fell from a normal value to near zero, indicating hyperthyroidism. Conclusions: Either of these parameters promise to be simple cost effective methods to differentiate between euthyroid and hyperthyroid conditions in patients with intact hypothalamic and pituitary function. The sensitivity and specificity of the T and P methods seems compatible with TSH measurement. An advantage of the methods is their simplicity; the only protocol change is to add a dynamic study to the existing static imaging protocol. Measurement of a standard is not necessary and there is no extra radiation dose to the patient. (author)