From "medical net":
If you have a thyroid scan, you will be asked to take a drink, or swallow a pill containing the iodine. You will then wait for the iodine to be taken up by the thyroid - usually about 4 hours. At this time you lie under a scintography camera, and it will take pictures which correlate directly to the amount of iodine taken up by the thyroid. This is done by counting the intensity and location of the gamma rays emitted by the radioactively labeled iodine. You then leave the hospital, and return in 24 hours to have a second scan performed in the same manner.
What if a US has already shown it to be a solid mass. Would the CT show hot or cold?
According to the definition, the CT scan can image both solid and soft tissue, therefore it is better for diagnosis of solid nodules or complex cysts.
Which scan will reflect a hot or cold nodule in the most effective way? MRI - CT or what?
The hot nodule is composed of packed thyroid cells, so in most cases the FNA is not required. If such nodule is subjected to "intensive" palpation, the person may develop perod of hyperthyroidism, don't know about FNA.