Target: the person's state or circumstances.
Source: a speaking human being, not explicitly named as the comparison source.
Associated attribute: a tongue, the organ through which speech is expressed.
The resemblance is the communication of meaning or emotion. A speaker expresses thoughts through words; observable circumstances can also communicate the person's actual situation.
The poem contrasts the speaker's literal voice of gratitude with the condition's imagined voice of complaint. The image makes outward circumstances a more powerful witness than the speaker's verbal profession of thanks.