Hyperthyroidism, coloured scintigram (gamma camera scan). The thyroid is an endocrine gland in the neck responsible for producing and releasing hormones that control the metabolism. Its activity can be studied by injecting radioactive iodine, as the hormones it produces are made with that element. The uptake of radioactive iodine by the gland can be measured by imaging it with a gamma camera, which detects the gamma rays the iodine emits. Here, the right lobe of the gland (on the left in the image) appears brighter and larger than the left, indicating it has higher activity. An overactive thyroid can lead to weight loss, fatigue and depression.