Unfortunately, he quickly lapsed into illness. The Caligula that emerged in late 37 demonstrated features of mental instability that led modern commentators to diagnose him with such illnesses as encephalitis, which can cause mental derangement, hyperthyroidism, or even a nervous breakdown (perhaps brought on by the stress of his position). Whatever the cause, there was an obvious shift in his reign from this point on, leading his biographers to label him as insane.