Can water softeners, be a contributing cause of arthritis and other illnesses, also in our pets, who drink that water?

There is no link between water softeners and causing arthritis. In fact most studies show that drinking softened water is good for you. Softeners have been in use since 1916 and in all that time there has never been any ill health claims against them.

Type water softeners and effects on arthritis into your search engine and you will find many reports.

Best wishes. 

RA (rhuematoid arthritis) is an autoimmune disease and was more likely caused by an illness or physical trauma. As for your cat, if you feed him (her?)dry food, use a formula that promotes urinary health (most major brands carry one). Try buying bottled water to give to your cat-there is less calcium (same as in softened water) and that is a big factor in renal/urinary health (too much calcium).  As for kitty's thyroid problems I am not sure (thyroid issues are related to iodine levels in the body). Has kitty been seen by a vet recently? Hope this has helped!

Arthritis translates basically as changes in the joints, and over time and over use of the body, certain joints will change in response to the physical stress. The Rheumatoid Arthritis your mother had is genetic, Osteoarthritis (that comes from normal aging and using the body) is not genetic and can be controlled by diet and exercise.

As far as your water softener, the water is "hard" when it is overloaded with heavy minerals, and the purpose of the softener is to take those away, so that the kidney and liver are under less stress. I do not know of any studies per se, but I can tell you that the body will react adversely to either extremes ie too many minerals in the water or too few.  It is likely that the softener is removing too many nutrients, some of which you (or your cat) may actually need to maintain bone and function in the body.

Check the filtering system and call the company and ask them how much they filter out.