A hand was a body part.
The Tenth Doctor's original right hand was cut off during a sword fight against the Sycorax leader. (TV: The Christmas Invasion) Jack Harkness later retrieved the hand and returned it to the Doctor in a jar in the year 100 trillion. (TV: Utopia) Later, when forced to regenerate, he healed himself and channelled the excess energy into the hand, a bio-matching receptacle. (TV: Journey's End) After the Daleks captured the TARDIS, they attempted to destroy it. Still inside during this apparent destruction, Donna Noble touched the energised hand's container. The regenerative energies within the hand combined with Donna's human DNA. The force of the reaction shattered the container, allowing the creation of the "Meta-Crisis Tenth Doctor". (TV: Journey's End)
The Thirteenth Doctor's hands were noticably smaller than her predecessor, causing the ring that he had worn to be released and fall to the TARDIS control room floor upon his regeneration. (TV: Twice Upon a Time)