Depending on diagnosis, the vet will most likely recommend fighting the condition with a combination of the following methods:
Vasodilator Drugs
Vasodilators will maintain the dilation of blood vessels. This will decrease the cat's blood pressure (making exercise easier) and stop it from developing circulation problems to its extremities.
Antiarrhythmic Drugs
An antiarrhythmic will keep all the chambers and valves of the heart contracting and expanding in the correct order. This will go a long way towards preventing the faulty valve from allowing blood to be pushed backward into the adjoining veins.
Diuretics
Diuretic drugs are key to preventing fluids from building up and impacting your pet's quality of life. This is done by promoting increased urine production, meaning that excess liquid that would once have pooled in the lungs will now be expelled from the body. This increased loss of water can also result in radical weight loss, so be sure to keep track of your cat's health before follow-up visits to the vet.
Note that these drugs are almost always delivered in pill form, and commonly are crushed into food to aid delivery.
In the event of a thyroid tumor, surgical intervention may be necessary to prevent it from spreading throughout the body.