All he [the assassin] does is smear an imperceptible amount of the thyroid hormone tri-iodo thyronine (which can induce hundreds of beat per minute heartbeats in a single milligram dose), on the inside of a wine glass before Breitbart shows up, and sets it aside (you know Breitbart orders wine, and even his favored brand and year, because it is in his surveillance file). Then, when Breitbart comes in (you know he is coming because he is covered 24/7 and you know his patterns), you take that glass and serve him his wine. 30 minutes later Breitbart is dead of a heart attack.
Nobody knows, because Tri-iodothyronine is produced naturally by the body, nobody ever tests for it, and even if they did in this case I am not sure whatever was in his system wouldn’t be bound out to receptors by the time Brietbart’s blood was drawn. Even if it tested high, a coroner is going to think of natural over-production like hyper-thyroidism or thyroid storm before he thinks poison.