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The John Thomas sign, also known as the Throckmorton sign, is a slang or joke term used in the field of radiology. It refers to the position of a penis as it relates to pathology on an x-ray of a pelvis. When the penis (visible on the x-ray as a shadow) points towards the same side as a unilateral medical condition such as a broken bone, this is considered a "positive John Thomas sign", and if the shadow points to the other side, it is a "negative John Thomas sign."
Studies have shown that the "sign" is no better than chance at identifying the location of a hip fracture. One such study concluded that "The use of John Thomas sign remains limited to introducing humorous atmosphere in orthopedic meetings as the sign is clinically unreliable." In those cases where the John Thomas sign is positive, it has been proposed that a person with a displaced hip fracture may try to lie on the injured side to immobilize the fracture and reduce pain; the penis then inclines toward the downward (injured) side.