In 1953, Julian P. Boyd wrote: "The use of nicknames and Christian names in ordinary discourse and in correspondence among cultivated adults in the America of Thomas Jefferson's time was almost non-existent, despite the evidence to the contrary in historical novels and dramatizations of the 20th century. This evidence is of recent manufacture."
While it is impossible to recover the ordinary conversation of Jefferson and his contemporaries, we can draw conclusions from surviving correspondence. Jefferson's almost invariable salutation was "Dear Sir" or "Dear Madam." The exceptions were his immediate family and the close friends of his youth. The letter he addressed in his early years was "Dear Page," Dear Nelson," or "Dear Fleming," and he was addressed "Dear Jefferson." In the later years all have become "Sir or "My dear friend."
We have found only one case of Jefferson's use of a nickname or first name of someone other than a member of his immediate family: his two earliest letters to William Fleming (1763 and 1764) begin "Dear Will." Even his brother-in-law Francis Eppes is "Sir" and his sister-in-law "Mrs. Eppes." And only the Italian Charles Bellini addressed Jefferson as "Tommaso," in Julian Boyd's opinion less from intimacy than from obsequiousness.
In correspondence with family members like Francis Eppes, Jefferson referred to his wife Martha as "Patty," while even to close friends like John Page she was "Mrs. Jefferson." She was also "Mrs. Jefferson" in Jefferson's own Memorandum Books. His daughter Martha referred to her husband in correspondence as "Mr. Randolph," and even in letters to her father. Presumably her mother expressed herself with similar formality. Jefferson usually ended his letters with the name: Th. Jefferson.