Basic patterns of simple hormonal control pathways. In each pathway, a receptor/sensor (blue) detects a change in some internal or external variable— the stimulus—and informs the control center (gold). The control center sends out an efferent signal, either a hormone (red circles) or neurohormone (red squares). An endocrine cell carries out both the receptor and control center functions.
Graves′ disease, the most common form of hyperthyroidism in humans. Tissue behind the eyes can become swollen and fibrous, causing the characteristic symptom of bulging eyes.