Goiter, Hyperthyroidism, Hypothyroidism -- another great one from Tracy

Robin <Eclectk1@...>

Again, from another bb, someone mentioned a pony that resides at a
zoo and is being treated for Cushings. They said the odd thing is
that his Thyroid is enlarged, and that he's had symptoms virtually
the reverse of diabetes... I posted wondering if Goiter was possible
in horses, and the following was Tracy's reply:

Goiter in horses is rare but possible. I remember reading years ago
that horses that lived where the feed is consistantly low in iodine
(strictly inland areas where the soil is low in iodine) can develop
goiter. Also, paradoxically, if a horse is exposed to too much iodine
the thyroid can swell the same as it does when the goiter is caused
by too little iodine. Iodine deficiencies are extremely rare in the
U.S. due to mineralized salt blocks that almost always contain small
amounts of iodine, and vitamin/mineral supplements which also contain

I did have one experience many years ago when a friend suggested that
I add a micromineral seaweed based supplement to my horses' diets and
I did so and followed the directions. One mare very quickly developed
some swelling at the throat which on palpation felt like her thyroid
was somewhat enlarged. At that point I called the company and got
them to send me a guaranteed analysis, which showed the major
micronutrient supplied by the product was iodine and it was in fairly
large amounts. Our hay is grown in the Colorado River Valley and
while it is low in most trace minerals, iodine is not a problem
because the area used to be under sea water many thousands of years
ago. In this mare's case the extra iodine was sufficient for her to
develop symptoms of goiter from an overabundance, in all likelihood
she was very well adapted to her native diet and didn't need as much
extra iodine as this product was supplying. So I stopped feeding the
supplement and within a week her thyroid went back to normal size.

As to how goiter develops, it is symptomatic of iodine deficiency or
excess (excess is more rare). Iodine is a necessary constituent in
the formation of the thyroid hormone T3 (triiodothyronine) which is
made from T4 (thyroxine) by the thyroid. When there isn't sufficient
iodine in the body the thyroid can't convert T4 to T3. The result is
an overabundance of T4 while there is an underabundance of T3, and
the symptom of swelling of the thyroid that we call goiter. Excess
iodine also results in goiter-like symptoms, though I'm not sure of
the actual mechanism in this case. Hypothyroidism that results from
iodine deficiency is common in developing countries but rare in the
U.S. and other developed countries because of iodine supplementation
(we put it in salt). Hypothyroidism that comes about from other
causes, Hashimoto's disease for example, usually does not present
with symptoms of goiter (although they can). This is because the
thyroid in these cases may not be producing sufficient levels of both
T4 and T3. Or in one type of disease the thyroid output of both
hormones is correct but the body builds anti-bodies to the hormones
themselves and destroys them before the hormones get a chance to do
their work.

As for the insulin situation seen in the pony mentioned above,
anytime you have an imbalance in one metabolic hormone it inevitably
leads over time to an imbalance in all of them. In this case
Cushing's disease causes the pituitary to overproduce both TSH
(thyroid stimulating hormone) and the cascade of hormones that
stimulate the adrenal glands to produce glucocorticoids. Too much
circulating glucocorticoids leads to insulin resistance
(hypoglycemia) which is what you are seeing in this pony right now.
Over time insulin resistance will lead to diabetes (hyperinsulinism),
they are the first and second steps in a chain reaction of metabolic
events. The treatment of feeding the pony in smaller meals and at
frequent intervals is an excellent treatment for insulin resistance.
They might go a step further and reduce or remove from her diet all
short chain carbohydrate sources, that means no grain and no sugars
of any kind, and increase her level of protein -- Dr. Ralston's work
on older horses and especially Cushing's horses has shown that older
horses need and can tolerate more protein in the diet than younger
ones, they also have poor tolerance of short chain carbohydrates.
Also if they can add Vitamin C to her diet without adding too much
sugar that would be helpful. The zoo might consider contacting Dr.
Ralston for a diet consultation.

One other thing about the pony in the above case, while it is very
rare in horses, hyperthyroidism, Grave's disease is seen in other
animals, and should be ruled out in her case. It is caused by either
a cancerous (rare) or benign overgrowth of thyroid tissue cells in
one or both lobes of the throid or in pockets throughout the body.
This leads to symptoms of hyperthyroidism (weight loss, high resting
heart rate, intense hunger). Nodules or swellings can often be
palpated on the thyroids of animals affected with Grave's disease,
unlike goiter in which the whole thyroid is enlarged. Although if
both lobes of the thyroid are involved it can feel similar to goiter.
Grave's disease is common in cats and is completely treatable with
radioactive iodine treatments or via surgery. I have never heard of a
horse being treated for Grave's but one assumes that it would be
possible, though quite expensive. (One of my cats had the disease and
it cost $800 to treat, she's fine now).

Tracy Scheinkman Misty Mountain Arabian Sport Horses Tucson, AZ