Thyroid scintigraphy: establishing a clinically useful normal range for 99mTc pertechnetate uptake.

Purpose: This study aims to establish a normal range for the thyroid uptake derived from ^99mTc pertechnetate scans. In particular, variations of uptake with TSH stimulation and other factors such as urinary iodine concentration are taken into account and compared with the calculation of a raw uptake value.

Methods: Clinical multicentric (center A, B and C) prospective study on 125 consecutive healthy patients undergoing thyroid scans for thyroid nodules. Normal functional thyroid status was assured by normal TSH, normal thyroid size, no thyroid antibodies and no symptoms of thyroid functional disorders. Calculations of raw Tc-uptake (uptake) and modified uptake values regarding current TSH value (uptake_TSH1), urinary iodine concentration (uptake_TSH1&uic), gland volume, age, smoking status, weight and tissue thickness ventral to the thyroid were performed.

Results: There is a positive correlation of thyroid uptake with TSH allowing for the calculation of a normalized uptake value (uptake_TSH1). The normal range for uptake_TSH1 compares favourable to that for raw uptake in that it yields a clear distinction from thyroid functional disorders. The additional normalization for urinary iodine concentration (uptake_TSH1&uic) may even improve the distinctive power whereas further normalizations such as for gland volume, age and others are not warranted by this study. The 95% CI of uptake_TSH1 for sites A, A&B, and A&B&C were 0.21%-2.06%, 0.22%-2.38% and 0.24%-2.40%.

Conclusion: A normal range for the thyroid uptake can be established with respect to the current TSH stimulation. This normalization (uptake_TSH1) overcomes the drawback of raw uptake by yielding a clinically useful parameter with obviously high distinctive power against functional thyroid disorders.