[NAME, COMPANY AND ADDRESS, ex.
Jill Smith
XYZ Inc.
1234 First Street
Suite 567
Anycity, Anystate  85245]

Dear [NAME, ex. John Smith], 

Thank you for taking the time out of your busy schedule to visit me in hospital during my illness.

It is gestures like yours which helped give me the strength to recover, and I really enjoyed seeing your friendly face there. 

Thank you again, [FIRST NAME, ex. John]

Sincerely,

[YOUR NAME, ex. John Jones]