Thyroid uptake versus thyroid salivary ratios: which is more reliable for post-operative thyroid functional assessment in patients with recurrent hyperthyroidism?

ABSTRACT

Tc-99m pertechnetate thyroid uptake has been introduced as a useful adjunct to measurement of hormonal levels in patients with recurrent hyperthyroidism after surgery for toxic goiter. A crude but relatively accurate estimate of thyroid function can be obtained by comparing thyroid to salivary gland activity at 20 minutes. We aimed in the current study to assess the value of Tc-99m pertechnetate thyroid uptake [TU] and thyroid-salivary ratios [both thyroid/salivary ratio TSR and salivary/thyroid ratio; STR] in diagnosis of post-operative recurrent hyper-functioning thyroid tissue. The current study included 114 patients presented for thyroid scintigraphy 3-9 months post subtotal thyroidectomy for toxic goiter [73 Grave's disease and 41 toxic nodular goiter]. Ten patients proved to have hypothyroidism were excluded from the study. All patients were subjected to clinical history and examination, thyroid hormonal profile, Tc-99m pertechnetate thyroid scan with calculation of TU, TSR and STR 20mjn after injection of the radiotracer. Thyroid uptake per unit weight of residual thyroid tissue was also calculated. TU value was considered normal if it is in the range of 0.3 to 3.75%.According to thyroid hormonal profile, patients were divided into two main groups, Group 1 Included 25 patients with hyperthyroid state. The mean value of TU was 10.2%. The mean value of TSR and STR were 11.75 and 11.1%. Twenty-three patients had both indices pointing to hyperthyroidism. The sensitivity of the thyroid uptake value for diagnosis of hyperthyroidism in this group of patients was 68%, while the sensitivity for both TSR and STR was 92%, with a statistically significant difference between both figures [p<0.001]. Group 2 Included the remaining 79 patients with normal thyroid hormonal profile. Their mean uptake value was 1.58%, while the mean value of TSR and STR were 2.5% and 57%. All patients in this group had normal thyroid uptake level. The specificity of TU for diagnosis of recurrent hyperthyroidism was 100%. versus 97.5% for both TSR and STR. The mean thyroid uptake per unit weight of residual thyroid tissue in group 2 was 6.4, which is significantly less than that reported for group 1 [19.5%] [p<0.001]. Thyroid uptake value has lower sensitivity [68%] than thyroid salivary ratios [TSR and STR] [92%] in diagnosis of post-operative recurrent hyperthyroidism with comparable specificity. So, TSR and STR are more reliable than Tc-99m pertechnetate TU in diagnosis of recurrent hyperthyroidism post operatively and may be applied in this group of patients for assessment of the functional status of residual thyroid tissue. Thyroid uptake per unit weight of remaining thyroid tissue is significantly higher for recurrent toxic state compared to euthyroid state; this may represent a new useful functional index, warranting further randomized study on much larger number of patients to verify its exact value