Ask the Nutritionist: Dr. Getty's Forum for Equine Nutrition
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All the best,
Dr. Getty
Hi Donna,
An enlarged thyroid gland can be caused by too much iodine (seen with diets that contain kelp or sea salt) or can be caused by too little iodine (which is rare). However, too much calcium, such as diets containing only alfalfa hay, can lead to a secondary iodine deficiency. White clover or vegetables belonging to the Brassica family (broccoli, cabbage, cauliflower) can also induce an iodine deficiency.
With an iodine deficiency, you would see poor skin and hair coat. And, as I mentioned, it is unusual to see in most horse diets.
Have your vet rule out a thyroid adenoma -- a benign growth that can occur in older horses.
I recommend you evaluate the amount of total iodine that your horse is receiving from her supplements. She should not get more than 3 to 4 mg of Iodine per day. If you are feeding sea salt, or over-feeding alfalfa, or supplementing calcium, changes should be made.
If you feel that more iodine is needed, give your horse iodized table salt (that you buy in the grocery store). You can offer it free choice, or add 2 tablespoons (divided between meals) in her meals.
Keep me posted.
All the best,
Dr. Getty 
Author of Feed Your Horse Like A Horse
Thank you for your advice...........great forum!
Donna
Where are you from? Bayfield, CO
Hi Dr. Getty,
Thank you so much for replying. My vet is scheduled for Monday to ultrasound and take a sample of the enlarged thyroid,and retest bloodwork is that what you would recommend? The Buckeye Gro'n Win has 4ppm Iodine (is that okay)? How do we know how much is in her hay/pasture? I just had an analysis done on the hay but I don't see Iodine listed? My new hay has alot more Alfalfa and I am trying to find some straight orchard grass hay to mix with it. Also you mentioned White Clover? I have ALOT of clover in my pasture and I am trying to get rid of it.....I also have a secondary problem (that is much more serious) but I didn't want to take up too much of your time but when this enlarged thyroid came up 30 days ago that is why I called the vet and he took blood and her insulin is really high so could these two things be connected? I will do ANYTHING for this mare............I have owned her since she was a baby..... 
Donna
Where are you from? Illinois
Hi Donna,
Yes, blood work will help, but T4 levels often show as low with an insulin resistant horse (elevated insulin) even though there is no thyroid problem. Keep me posted on what your vet says and it may be worth it for us to visit personally over the phone. So, please consider making an appointment.
4 ppm in your feed translates into 4 mg of iodine per kg of feed. If, for example, you feed 2 lbs, that is equivalent to 0.91 kg (2 divided by 2.2). So, that feed would provide 3.64 mg of Iodine. That is a considerable amount and therefore, you should avoid adding iodine from any other source. Iodine in hay and pasture, even legumes, is very minimal and not enough to even list in the Nutrient Requirements for Horses manual.
So, keep me posted and let me know if I can assist you further.
All the best,
Dr. Getty
Author of Feed Your Horse Like A Horse
Thank you so much for replying. My vet is scheduled for Monday to ultrasound and take a sample of the enlarged thyroid,and retest bloodwork is that what you would recommend? The Buckeye Gro'n Win has 4ppm Iodine (is that okay)? How do we know how much is in her hay/pasture? I just had an analysis done on the hay but I don't see Iodine listed? My new hay has alot more Alfalfa and I am trying to find some straight orchard grass hay to mix with it. Also you mentioned White Clover? I have ALOT of clover in my pasture and I am trying to get rid of it.....I also have a secondary problem (that is much more serious) but I didn't want to take up too much of your time but when this enlarged thyroid came up 30 days ago that is why I called the vet and he took blood and her insulin is really high so could these two things be connected? I will do ANYTHING for this mare............I have owned her since she was a baby.....
DonnaWhere are you from? Bayfield, CO