John Thomas sign

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Fracture of the left femur with the shadow of penis pointing there

The John Thomas sign,[1] also known as the Throckmorton sign,[2] is a term that refers to the position of a penis as it relates to pathology on an x-ray of a pelvis.[3]

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.

The sign is employed as a humorous aside.[2] However, some analysis of its validity has been performed, with a mistaken assumption being a possible explanation: rather than the penis actually pointing to one side, "the anteroposterior view of the pelvis is in fact an oblique view, which shows the fracture and the penis shadow on the same side."[3][4] Genital asymmetry correlates with handedness,[5] and a relationship between handedness and injury has been proposed as a mechanism for the sign.[3]


  1. ^ "Note: "John Thomas" is an English language euphemism for "penis"
  2. ^ a b synd/3050 at Who Named It?
  3. ^ a b c Thomas MC, Lyons BD, Walker RJ (1998). "John Thomas sign: common distraction or useful pointer?". Med. J. Aust. 169 (11-12): 670. PMID 9887926. 
  4. ^ Mouzopoulos GJ, Stamatakos MK, Mouzopoulos DJ (2007). "Does penis radiological shadow indicate the side of hip fracture?". J Postgrad Med 53 (3): 217. doi:10.4103/0022-3859.33873. PMID 17700004. 
  5. ^ Bogaert AF (January 1997). "Genital asymmetry in men". Hum. Reprod. 12 (1): 68–72. doi:10.1093/humrep/12.1.68. PMID 9043905.