IODINE IN THE
HORSE TOO MUCH OR TOO LITTLE
Iodine is an essential nutrient for reproduction and normal
physiological function in the horse. Thyroxine contains iodine,
and this hormone along with triiodothyronine (T3) has powerful
effects on the overall health of the horse. These hormones
influence nearly every process in the body, from heat regulation
and feed utilization to proper bone growth and maturation.
Nearly 75% of the iodine in an animal's body is in the thyroid
gland. Iodine deficiency may result in goiter as the thyroid
gland enlarges in an attempt to produce thyroxine. In the
horse, goiters often occur in the foal at birth. Foal goiter
may result from a deficiency in iodine in the mare's ration
during pregnancy or it may be caused by a goitrogenic substance.
Symptoms of iodine deficiency may be a stillborn foal or a
very weak foal at birth that cannot stand and nurse. The foal
may also have a rough haircoat, contracted tendons, angular
limb deformities or other abnormal bone development. A Russian
study (Kruzkova, 1968) indicated that mares which had shown
anovulatory cycles responded to iodine supplementation.
While iodine deficiency is the primary cause of goiter in
foals, excessive levels of iodine may also cause this condition.
The maximal tolerable dietary concentration of iodine has
been estimated to be 5 mg/kg (PPM) of dry matter (NRC,1980),
equivalent to 50 mg of iodine/day for a horse consuming 10
kg of dry matter daily.
The horses most
sensitive to high iodine levels are foals from mares who are
supplemented with high levels of iodine. Iodine is concentrated
across the placenta and in milk so that the fetus and nursing
foal receive much higher concentrations than are present in
the mare's ration. Therefore, goiters may be present in newborn
foals while sparing the mother A dietary intake of 83 mg I/day
is the lowest level reported to have caused goiter in a horse
more mature than a suckling foal (Drew et al, 1975). Baker
and Lindsey (1968) reported that goitrous foals were born
on three farms which were feeding mares high levels of iodine.
The incidence of goiter was proportional to the level of iodine
fed and equaled 3% on one farm feeding 48-55 mg I/day, 10%
on a farm feeding 56-69 mg I/day and 50% on another farm feeding
288-432 mg I/day. A neighboring farm which did not have any
goitrous foals fed iodine at a rate of 6.3-7 mg I/day.
Sipple (1969) reviewed
a case in which 11% of the foals born on a farm had goiters.
Analysis of the diet revealed that the mares received between
160-400 mg I/day. Coincidentally, the author discovered that
the manager of this farm was the brother of the manager of
one of the farms in Baker's study in Florida. apparently,
the Florida horseman had prescribed the same iodine supplement
for his brother's horses 1,000 miles away. Drew et al (1975)
reported that on one stud farm in England
four foals were born with greatly enlarged thyroids and leg
weakness. One mare also had an enlarged thyroid. Feed analysis
showed that the mares had received 83 mg I/day from a proprietary
feed during pregnancy. The year before the introduction of
this proprietary feed, the mares received a vitamin / mineral
supplement which supplied about 12 mg I/day and there was
no problem with goiter on the farm. The results of these studies
are summarized in Figure 1. It appears from these reports,
that around 50 mg of dietary iodine is required in the daily
rations of mares to produce any incidence of goiters in their
foals. One other study (Driscoll et al, 1978) reported goitrous
foals from mares receiving 35 mg I/day. There is some question,
however, about what levels of iodine the mares in this study
actually received. The authors reported that the mares were
given 12 ounces per day of a supplement which was reported
to contain 58 PPM iodine. The guaranteed analysis on the product's
label stated that it contained 340 PPM iodine and independent
analyses of the same product revealed that it contained at
least 580 PPM iodine, a level 10 fold higher than reported
in the paper. Using the manufacturer's guarantee, the mares
would have received a total of 131 mg I/day and according
to the independent analyses, a total of 212 mg I/day. These
levels are within the ranges reported to produce goitrous
foals in other studies.
Toxic dietary iodine concentrations may result from adding
excessive supplemental iodine, such as from ethylenediamindihydroiodide
(EDDI), to concentrates or from using feedstuffs high in iodine.
A common feedstuff that may contain excess iodine is kelp
(Laminariales), a specific family of seaweeds that may contain
as much as 1,850 PPM iodine (NRC,1989). Unfortunately, people
have a tendency to classify all seaweeds as kelp just as the
layman might consider every breed of horse a Thoroughbred.
There are numerous other specific seaweeds that contain considerably
less iodine than kelp. SOURCE®, a dietary micronutrient
supplement for horses, CONTAINS NO KELP. It is made from a
blend of certain other dehydrated seaweeds including Fucaceae,
Palmariaceae, Gigartinaceae, Bangiaceae, and Ulvaceae. The
seaweeds of these Families which are utilized in SOURCE contain
considerably less iodine than kelp. In addition, all components
used in SOURCE are analyzed for iodine, as is the final product
blend. SOURCE, at the recommended feeding level of 1/2 ounce
per 1,000 lbs. body weight, provides approximately 9 mg of
iodine. This amount of iodine is well below the levels reported
to cause problems in horses. In fact, many of the "control"
farms reported in the literature fed this level of iodine
to their horses with no suggestion of iodine toxicity in either
the mares or foals.
Goiters in horses can be cause by either too much or too little
dietary iodine. It is of paramount importance that the actual
dietary intake of iodine and the possible presence of goitrogenic
substances be established before treatment is administered.
All too often, additional iodine is given to goitrous horses
when the diet already contains excessive iodine. The mare's
ration should be evaluated in instances of foal goiter since
iodine can be concentrated in the fetus and in milk. A common
culprit in many cases of iodine toxicity in horses is kelp.
SOURCE contains no kelp and at its recommended level of intake
provides a safe and effective amount of dietary iodine which
will safeguard against iodine deficiencies in horses.
Baker, H.J. and
J.R. Lindsey. 1968. Equine goiter due to excess dietary iodine.
J. Am. Vet. Med. Assoc. 153:1618.
Drew, B., W.P.
Barber, and D.G. Williams. 1975. The effect of excess iodine
on pregnant mares and foals. Vet Rec. 97:93
Driscoll, J. et
al. 1978. Goiter in foals caused by excess iodine. J. Am.
Vet., Med. Assoc. 173:858
Kruzkova, E. 1968.
Mikroelementy i vos proizvoditel-'naja funkeija kobyl. Tr.
Vses. Inst. Konevod-stvo. 2:28 (as cited in Nutr. Abst. Rev.
Council. 1989. Nutrient Require-ments of Horses. Washington
D.C.: National Academy Press.
Council. 1980. Mineral Tolerance of Domestic Animals. Washington
D.C.: National Academy Press.
Sipple, W.L. 1969.
A Veterinarian's Approach to Stud Farm Nutrition. Eq. Vet.