I understand that Penelope has multiple health issues including hypothyroidism, seizures, and allergies.
Does she have a history of an ear infection or is she suffering from one now?
From your brief history, her age and incoordination I believe that your pup may have vestibular disease.
Is her head tilted to the right or left?
When she tries to walk is she circling or leaning to one side or walking very slowly with a wide stance and then leaning and falling over?
Does she have rhythmic back and forth or circular eye movement (like the dog in the video in this link: http://www.youtube.com/watch?v=NaB7OJRLVQ4 ) ?
If the answer to these questions is yes then she may have vestibular disease. Vestibular disease is a malfunction of the balance system, either a problem in the inner ear itself or in the nerves that take information to the brain or in the brain itself. Episodes often come on suddenly.
With vestibular disease she can get very dizzy and as such have trouble with coordinated movement. They will often fall or lean to one side, staggering when trying to walk or get up.
If she has a history of an ear infection that makes vestibular disease even more likely. It is possible that the ear infection has spread into the inner ear affecting her balance system.
There can be several causes of vestibular disease. They range from very benign causes such as idiopathic (meaning we don't know the cause but they resolve on their own with supportive care) to middle ear infections or polyps, brain infections (bacterial, fungal or viral) or even a primary brain lesion such as a blood clot, bleeding or a tumor.
If we cannot identify a cause then we will often treat the patient symptomatically (anti-nausea drugs, anti-inflammatories and possibly antibiotics if an ear infection is a concern) as most dogs do get better with supportive care.
Her prognosis if this is caused by a lesion outside the brain is very good in most cases.
We may check bloodwork to make sure organ failure or low thyroid hormone are not the cause of her symptoms. That is especially important for her because we know she has poor thyroid function. Perhaps her medication needs to be adjusted.
If she isn't improving in 3 to 5 days then blood tests should be done to make sure all is well internally.
I do recommend that she see her veterinarian for an exam because if this is related to an inner ear infection she should be on systemic antibiotics.
In the meantime there are things that you can do at home.
You can give Gravol also known as Dramamine (dimenhydranate) to control nausea, which is also used for carsickness. The dose is 4mg to 8mg per pound of body weight every 8 hours. Side effects are mild sleepiness and dry mouth.
Or you can try Benadryl (diphenhydramine only, do not use products with acetaminophen or decongestants as they are toxic for dogs) at a dose of 1mg per pound of body weight orally every 8 hours. Side effects are sedation and dry mouth as well.
To stimulate her appetite if that seems off start a bland diet of 1/3 boiled, lean hamburger (or boiled, white skinless chicken), all fats and juices drained off mixed with 2/3 boiled, plain white rice. Add warmed low salt chicken or beef broth to get additional fluids into her and make the food more palatable. Feed several small meals a day.
You may need to hand feed her and offer her water or ice cubes to lick as she may have difficulty negotiating eating and drinking on her own.
If she still won't eat even after medication to decrease her dizziness and offering a bland diet then to help with nausea at home you can give either:
1) Pepcid ac (famotidine) at a dose of one 10mg tablet per 20 to 40 pounds of body weight every 12 hours
2) Prilosec (omeprazole) at a dose of one half of a 20mg tablet per 20 to 40 pounds of body weight every 24 hours
These are acid reducers and should help her feel a little less nauseous. They are quite safe and can be used for several days if necessary.
If she starts vomiting or refuses to eat for a couple days she may need hospitalization for supportive care, fluids and injectable medication for nausea.
If after a week's time there is no improvement or she is worse then she needs diagnostics to try and identify the cause. Serious central nervous system (the brain) causes are more likely and thus prognosis is much more guarded.
Things such as an MRI or spinal tap are indicated at that point.
If she is not improving a referral to a neurologist is best as they can perform this advanced testing.
Please see this link if you would like to read more about vestibular disease:
Please let me know if you have any further questions.