"I have known SEVERAL farms that had mares with a thyroid imbalance and every one of them produced a dwarf foal. The mares were not on thyroid meds during the time of pregnancy. When the horses did not resettle, it was discovered it was due to low thyroid."
There are certain types of hypothoidism that when occuring in the mother during pregnancy will cause certain developmental retardations including growth and fertility. The links from NCBI are vast, I chose to highlight the two links below that you can look at to chew on awhile. The two OMIM links bring up cases that the resulting babies showed dwarfism characteristics but were either lethal or nonfertile, meaning the dwarf could not reproduce, this is definitely not the case in the Miniature, because they can.
To try to critically examine your concern. Of those cases you have knowledge of about hypothyroid mares that have had dwarfs, have ALL the foals been dwarfs from ALL of those hypothyroid mares??
And your answer is -- "All of these mares had had previous normal foals prior to the thyroid going out of balance."
How do you know if the mares were hypothyroid before with the normal foals?? Were the mares tested for hypothyroidism during the gestation of the normal foals?? Wre they tested for hypothyroidism during the gestation of the dwarf foals?? That would be the ONLY way to correlate the dwarfism is caused by the hypothyroidism in the mares. AND hypothyroidism would have to occur every time in the mares to produce a dwarf everytime, and ontop of that the dwarfs would need to have very similar characteristics.
How old are these mares now that they are hypothyroid?? Hypothyroidism can be late onset due to age, known to occur in older and geriatric horses.
Since the mare would have a great influence over the development of the foal with regards to thyroid levels in the fetus during the first few months, and if those all of those mares are hypothyroid then every foal from every hypothyroid mare has to be effected. I really believe it is suspect that hypothyroidism is causing all the dwarfisms, especially multiple types. Also, the dwarf cases that I have been able to follow very closely in large breeding populations follow perfect percentages for autosomal recessive inheritance. I will say very few mares that have produced a dwarf, that I have known for a long time through mutiple foals, have shown hypothyroid characteristics, ie conception problems. On the contrary, quite a few carrier mares are prolific producers. That I know is not the case for hypothyroid mares, or hypothyroid mammals in general.
I have not tested blood from all mares that have produced dwarfs, heck I am lucky just to get a dwarf sample let alone enough blood from the mares to do any hormone testing. But again, for these dwarfs to be all over the world and all pedigrees, I would find it highly suspect for all these types to be caused by hypothyroidism alone.
Here is hypothoirism and dwarfism from NCBI, this site has links to just about any scientific database in the world
http://www.ncbi.nlm.nih.gov/sites/gquery?t...hyroid+dwarfism
Specifically I went to OMIM link below for the two links which deals human genetics and inheritance and comparing to animals
http://www.ncbi.nlm.nih.gov/sites/entrez?d...roid%20dwarfism
And this is the OMIA link from the NCBI link above, involving all animals including humans
http://www.ncbi.nlm.nih.gov/sites/entrez?d...roid%20dwarfism
"Has there been any study of these two things possibly being linked or a 'cause and effect' thing? Or is this coincidental?
There have not been any in horses but the links above will give you a lot of info to chew on for awhile.
John
