General:
* Suggested
* Implied
* Hinted
* Indicated
* Signaled
* Alluded to
* Whispered
* Suggested
* Implied
* Insinuated
* Proposed
Formal:
* Communicated
* Conveyed
* Declared
* Announced
Informal:
* Let on
* Dropped a hint
* Gave a heads-up
The best synonym will depend on the specific sentence you are using. For example, "He intimated that he was unhappy" could be rewritten as "He suggested that he was unhappy" or "He hinted that he was unhappy."
Consider the level of formality and the desired tone when choosing the best synonym.