WOW a lot of soloxine questions! Hopefully I can help clarify some of the questions.
The thyroid gland is responsible for metabolism. In simple terms, a lot of thyroid hormone leads to fast metabolism and very little of the hormone leads to slow metabolism. People will get both extremes of thyroid disease: too much thyroid (Hyperthyroidism) and too little thyroid (Hypothyroidism). Cats will almost exclusively get Hyperthyroidism and dogs will get almost exclusively Hypothyroidism.
Hypothyroidism has a lot of secondary changes to the body but luckily does not come with the serious life threatening problems of hyperthyroidism. One of these changes is increased lipidemia (increased blood fats, cholesterol and triglycerides). This can lead to lipid corneal deposits. Another eye problem associated with hypothyroidism is dry eye (KCS). In general, it is accepted that thyroid supplementation (soloxine) will lead to improved ocular health. So with the last question - maybe ask your vet but there may have been a miscommunication - hypothyroidism is more apt to cause lipid deposits that soloxine.
So lets back up a bit. How do we treat hypothyroidism? This is a lot easier than hyperthyroidism. When the body does not make enough thyroid hormone, what do we do? We just supplement it. The medication is levothyroxine. The brand name is Soloxine, all others are generic brands such as Thyrozine, Thyrosyn, etc... Is brand name important? The answer is sometimes but we can talk about that later if people are interested.
The great things about thyroid supplement - 1) works well, 2) usually low side effects, 3) inexpensive and 4) often chewable. The wonderful things about dogs and thyroid supplement is the canine's ability to shed any excess thyroid supplement into the urine. It is difficult to overdose and cause problems with this medication. (Side story: I inherited a case who was on over 10 times a dose twice daily for 6 months - I was so scared for the dog, I immediately referred the dog to the specialists to look for potential damage! the specialists had never seen a case like that and were interested to see what could be wrong. Especially with cats - the main side effect is high blood pressure which will damage the retinas (not the corneas), the kidneys and heart. Ophthalmologist evaluation of the eye - no problems. Ultrasound of the kidneys and heart - no problems. Blood work - no problems. Blood pressure - no problems. Dog was as normal as could be. )
With that said, I have had many client that do notice some behavioral changes. With increased metabolism, the brain seems to race a bit faster as well. My belief is that many of the signs would have been there if the dog only had a 'normal thyroid' but the low thyroid can 'numb' a lot of behaviors. On the other side, remember we are supplementing a hormone that should already be in the system. Hormones are powerful things and the body usually is secreting small amounts on a minute by minute, in fact second by second dosing. We on the other hand supplement it once or twice a day! When we take blood samples, the thyroid supplement after being given, peaks in the blood and then slowly decreases. During this peak, it would make sense that some additional behaviors may be seen as they may have a slight 'rush' - similar to an energy drink effect. (in fact it is common to use thyroid supplements with illegal steroid use as a method to help 'boost energy').
So what is the sum of all of this? Thyroid supplement is considered very safe for the average dog. There may be some side effects related to the rise and fall of metabolism rates. As long as the blood levels are looking fine, thyroid supplement is thought to do more good than its risks. Of course, this supplement should ONLY be used in dogs that are TRUE hypothyroid canine patients. Furthermore the blood levels and clinical evaluations should be made by routinely by the veterinarian.
If you are concerned that your pet may have some side effects from thyroid supplement what can you do? Talk to your veterinarian. They may want to run blood tests to evaluate thyroid levels to make sure they are within normal limits as well as to look for other common endocrine disorders that are commonly seen with hypothyroidism. If we are worried that the level is too high - then we may consider blood pressure testing, urinalysis, etc...
I noted that one of the posts made mention of heart disease. This is one of the disorders that thyroid supplementation needs to be evaluated carefully. Even though dogs can get rid of extra thyroid easily - we never want to push it with pre-existing diseases, especially cardiac.
Before I start putting people to sleep, I will stop. I hope this lessened the fears of thyroid supplementation which I believe has improved the quality of life for millions of pets around the world.