Thyroid uptake measures the relative amount of radioiodine (or radioactive iodine) that is taken up by the thyroid and indirectly reflects the thyroid function status: Euthyroid, hyperthyroid or hypothyroid.The normal range (euthyroid) of thyroid uptake depends on the iodine supply. While in US, a value of 15-35% is generally accepted as normal, in areas with low iodine in drinking water (e.g., Germany and Austria), normal uptake can be as high as 50%.
Radiotracers: 123-I (200-400 μCi) or 131-I (50 µCi)
The much lower dose of 131-I compared to 123-I used for thyroid uptake is because of the beta radiation associated with 131-I and “mini therapeutic” effect it would have if higher doses such as 400 µCi would be administered.
Thyroid scan gives a picture of the distribution of radiotracer and estimation of the size of the gland. Since thyroid scan is (in contrast to ultrasound, CT or MRI) not “scaled”, it cannot be used for size measurement. However, if a SPECT/CT of the thyroid (i.e., neck) is performed, the fused CT images are scaled, and can be used to exactly measure the size of the entire gland or a specific area such as a nodule.
Case Examples:
Thyroid ultrasound demonstrates two nodules with isoechoic to hypoechoic areas. The smaller nodule is in the left lobe and the larger nodule in the right lobe.
Planar thyroid scan performed with 400 μCi 123-I, demonstrates a faintly visualized cold left nodule and a slightly more suspicious right nodule, correlating with ultrasound findings. Thyroid uptake, measured with a probe immediately before imaging, was 17.9%, normal.
SPECT/CT of the thyroid demonstrates the two cold nodules, correlating with hypodense nodules on CT.
