What is meant by a hot nodule and a cold nodule?

Determining whether a nodule produces too much or too little hormone (in other words, if it is functional or not) is possible by means of measuring the thyroid hormone levels in blood (fT3, fT4 and TSH basal) and thyroid scintigraphy.

A hyperfuncional nodule means that the nodule shows increased hormone production which presents itself as a hyperactive (hot) area on thyroid scintigraphy. On the other hand, the scintigraphic diagnosis of a "cold" nodes means that the hormone production in the nodule is weak or not present at all and thus the nodule shows itself on the scintigraphy as a cold area that stores too little of the injected substance.