John Thomas Sign / Thockmorton Sign | Radiology Signs

The John Thomas sign (also called the Throckmorton sign) refers to the position of the penis on a pelvic X-ray in relation to a unilateral pelvic or hip pathology, such as a hip fracture. It is considered “positive” when the penis points toward the side of the pathology and “negative” when it points away. This phenomenon is thought to result from a patient’s unconscious attempt to lie on the injured side to reduce pain and immobilize the fracture, causing the penis to incline downward toward the affected side. A 2019 systematic review and meta-analysis found this sign has no reliable diagnostic value and is no better than chance at localizing pelvic or hip injuries — it is regarded as a humorous, non-validated eponym rather than a clinically useful finding.

Why is it called so?

“John Thomas” is a colloquial British euphemism for the penis, used because the sign describes the orientation of the penile shadow on pelvic X-rays relative to ipsilateral pathology. The alternative eponym, “Throckmorton sign,” is named after Thomas Bentley Throckmorton (1885–1961), an American neurologist — the name has since become attached to this radiographic curiosity, distinct from the unrelated “Throckmorton reflex” (a neurological pronator sign) also named for him.

Pathophysiology

The sign arises due to patient positioning and unconscious protective behavior — patients tend to lie on the injured side to mitigate pain and stabilize the injury, which causes the penis to point downward on the side of injury. This positional effect is projected radiographically as the shadow of the penis pointing toward the abnormality on the pelvic X-ray. Because this is a positioning artifact rather than a true pathophysiologic finding, it has not held up as a reliable diagnostic sign in formal evaluation.

Alternative names: Throckmorton sign

Other associated named signs: None

References

1. Gerber L, et al. The ‘John Thomas’ sign and pelvic fractures-Fact or humorous myth?: A systematic review and meta-analysis. J Clin Orthop Trauma. 2019;10(1):102-105.