There are many things that can cause the throat of a dog to swell. All of them require prompt veterinary attention. Swelling in the area of the throat can compromise swallowing and breathing with disastrous results.
Allergic reactions to insect bites or vaccines may cause sudden swelling of the throat. Untreated allergic reactions can progress to shock, suffocation, and death.
Trauma to the throat can cause swelling. Sudden jerks to the collar or other blunt forms of trauma may lead to the symptoms you describe. Penetrating trauma can occur when dogs impale themselves or chew on sharp objects, leading to swelling in the affected area.
Infections in the throat may lead to swelling. Wounds from fights may cause abscesses (severely swollen infected areas). Foreign objects such as foxtails can lodge underneath the skin or in the trachea or esophagus, leading to infected, swollen areas.
Problems with salivary glands sometime lead to throat swelling.
Unfortunately, cancer is a common cause of swollen throats in older cats and dogs. Lymph nodes in the throat may be infiltrated with cancer, causing them to enlarge.
Thyroid disease is common in eight-year-old dogs. However, it does not lead to swelling of the throat.
Your dog should go to the vet. Hopefully the problem is not serious.