Iodine Toxicity 
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.

Dietary Iodine
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.

Conclusions
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.

References

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. 39:807, 1968).

National Research Council. 1989. Nutrient Require-ments of Horses. Washington D.C.: National Academy Press.

National Research 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. J. 1:203

A: 6-7 mg I/day. No suggestion of iodine toxicity in mares or foals. Baker and Lindsey (1968).
B: 9 mg I/day. Level of iodine supplied from SOURCE®.
C: 12-14 mg. I/day. No suggestion of iodine toxicity in mares or foals. Drew et al (1975).
D: 83 mg I/day. Lowest reported level of dietary iodine shown to cause goiter in a horse more mature than a
suckling. Drew et al (1975).
E: 48-55 mg I/day. Goiters in 3% of foals. Baker and Lindsey (1968).
F: 56-69 mg I/day. Goiters in 10% of foals. Baker and Lindsey (1968).
G: 160-400 mg I/day. Goiters in 11% of foals. Sipple (1969).
H: 288-432 mg I/day. Goiters in 50% of foals. Baker and Lindsey(1968).

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