Thumb sign in lateral X- ray of neck seen in -
In epiglottis: A plain lateral soft tissue radiograph of neck shows the following specific features:
Thickening of the epiglottis—the thumb sign
Absence of a deep well-defined vallecula—the vallecula sign.
ALSO KNOW 
Steeple sign i.e. narrowing of subglottic region is seen in chest X-ray of patients of laryngotracheobronchitis (i.e. croup).
Epiglottitis
Internal haemorrhage
Saccular cyst
Ca epiglottis