Defense would have to look at the prognosis before the overdose, and compare how that had changed after the overdose. Whether the patient was admitted because he was end-stage would seem to matter a lot as far as damages go. A wrinkle here is that one of the drugs used to treat HF (i.e., beta blocker) might very well mask the effects of excess thyroid hormone - such as rapid and/or irregular heart rate.
Another possible factor might be if the Pt has diminished ability to convert the mostly inactive T4 to the active T3 - therefore an OD would not be as harmful.
(A little wrinkle is that hypoT might cause HF, though that is probably not relevant nor provable.)