Unlike fevers, hyperthermia occurs when a dog becomes overheated due to external temperatures being too high, or due to overexertion. The most common causes of hyperthermia in dogs include:
Excitement or Fear
Dogs who are trembling or shaking or panting from fear or excitement may have a mild elevation in body temperature.
Examples would include dogs with thunderstorm phobias, or even dogs with phobias of going to the vet or groomer. Overweight dogs, those with a big thick coat, and those with brachycephalic noses (pugs, bulldogs, etc), are more prone to becoming overheated for these reasons.
Diagnosis is usually based on the circumstances, a physical exam, and sometimes further tests to rule out other causes. Prognosis is good so long as you can remove them from the stressful situation. To prevent this issue, talk to your vet about prescribing your dog anti-anxiety medications prior to known stressful events.
Heat Stroke
Heat stroke is a life-threatening condition most often caused by dogs being inadvertently left in a too-hot environment. This includes being left in a car or outside without access to shade and cool water. It can happen to any dog, but dogs with thick coats, who are overweight, and who have brachycephalic conformation (squished noses) are especially prone.
Excessive panting and drooling are the most common symptoms of heat stroke, but you may also notice reddened gums, vomiting, diarrhea, loss of consciousness, uncoordinated movement, and even collapse.
Overexertion
Most dogs are good at slowing down when they’re tired, but young dogs, and those who are really fixated or focused on an activity (say playing fetch or jogging with their owner) can overdo it. This can become dangerous, especially if weather conditions are warm.
Seizure
When a dog seizes, their muscles contract and relax rapidly. If a seizure is prolonged (more than a minute or two) this can lead to an elevated body temperature. Treatment of this will involve controlling seizure activity first and foremost. A seizure lasting more than a few minutes is a reason to get your dog to the emergency room ASAP.