Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you today.
I am very sorry to hear about your lass's untimely passing.
In regards ***** ***** history, I would be most concerned that she had developed a type of heart disease called hypertrophic cardiomyopathy
(HCM). This is commonly a secondary issue in poorly control thyroid kitties.
HCM is a condition where the heart beats so fast (in these cats due to their thyroid disease) that the muscle of the heart walls become too thick. These thick walls reduce the inner space in the heart, which prevents it from filling as much as it once did. And if a heart cannot take up as much blood to push through per heartbeat, this leads to increased strain on the heart, progression of the disease and changes in the circulatory pressures. And it is the latter that causes fluid to leak from the blood vessels and cause it to pool in the chest.
Just to note, these cats do not often cough because unlike other heart diseases, the heart itself does not get bigger (since it is enlargement of the heart that pushes on the trachea to cause coughing).
Overall, based on what you have told me about Zoe, I would be most suspicious that she had developed HCM secondary to her thyroid disease. And I do have to note that cats are notorious for hiding the development of these conditions (since they modify their lives to avoid showing any respiratory elevation, exercise intolerance or fainting we might see in dogs) until they get to a sudden state of not being able to cope with the level of heart failure they have.
Please take care,