My wife has graves disease (well controlled) and seasonal allergies. Does her condition preclude the use of antihistamines? Claritin (loratadine) 't enough.

No, it doesn't. If the graves disease is well controlled (an autoimmune thyroid disorder), then treatment of other conditions such as allergies is not a problem. Perhaps, i would avoid decongestants such as pseudo Ephedrine that are often added to the antihistamine, because it cause rapid pulse, palpitations. If Claritin (loratadine) not helpful, worth trying others, such as zyrtec, allegra, allerest, (but not with the 'd').
Antihistamines. Claritin (loratadine) is a 2nd generation (non-sedating) antihistamine. If it doesn't affect your wife's graves ds control, a different 2nd generation one may work better for her without causing problems; Allegra is its name; there is also prescription clarinex, which may work better than claritin (loratadine). Zyrtec is a newer antihistamine but may not be as safe. Suggest discussing problem with endocrinologist.