Iodine - Nutrition

What is Iodine?

Iodine, like selenium, is a trace element that is required in only very small quantities. In nature its most common state is in form of salts of the iodide anion (negative ion) I^1-, such as potassium and sodium iodide, while some sources contain salts of the iodate anion IO^3-.

Why is it Important?

Iodine is essential because it is a major constituent of the thyroid hormones thyroxine (T₄) and 3,5,3’-triiodothyronine (T₃), the latter being considered the active form of the hormone.

Roles in the Body

Thyroid hormone is crucial for the regulation of cell activity, growth and development in growing animals and for the regulation of metabolic rate in adult animals. This means that iodine is an essential component for intermediary metabolism, reproduction and thermoregulation. Between 70-80% of total body iodine is concentrated in the thyroid gland, the remainder is present in blood. Iodine is absorbed primarily as the iodide from the gastrointestinal tract and then distributed throughout the body. In the thyroid iodine is converted by a series of reactions to T₄ and T₃, which effectively represent the active biological forms of iodine. Iodine is also concentrated in the kidneys and the main route of excretion is via the kidneys in the urine.

Consequences of Iodine Deficiency

Dog:

There are few studies on iodine deficiency in dogs. The main effects observed have been specific indicators of iodine or thyroid status rather than general clinical signs:

1. Thyroid changes: In a long-term feeding trial of dogs receiving low dietary levels of iodine (0.3 mg/kg dry matter – DM), thyroid glands became hyperplastic and hypertrophic during the first 9 months. Significant increases in the thyroid uptake of radiolabelled iodine were also reported. However, the dogs seemed to adapt as the study progressed beyond 9 months because the thyroid glands eventually returned to normal size and appearance.
2. Serum levels of thyroid hormones: When adult beagles were fed very low intakes of iodine (50 or 20 µg/day, equivalent to a dietary iodine content of about 0.27 and 0.11 mg/kg DM respectively) marked reductions in serum T₄ but not T₃ were observed.
3. 'Goitre and other clinical signs: In dogs fed low iodine levels primarily as a result of receiving an all-meat diet, signs consisted of goitre (enlargement of the thyroid gland), weight gain and adverse effects on hair condition.

Cat:

There is little information on iodine deficiency in the cat but one study identified changes to the thyroid gland linked to a low iodine diet.

1. Thyroid changes: Kittens aged 10 to 14 weeks were fed a diet of raw beef or sheep heart for 8 weeks or more; this diet contained only 0.45 mg iodine/kg DM. No clinical signs were observed but the thyroid glands from these kittens were enlarged and showed histological changes associated with iodine deficiency. Kittens receiving a dietary iodine level of 2.2 mg/kg DM remained normal but a diet containing 1.1 mg/kg DM was still associated with some signs of thyroid gland hyperplasia.

Toxicity

There are various clinical signs associated with excessive iodine intake including changes to the skin and hair, and bone abnormalities. Goitre – normally a feature of chronic iodine deficiency – may also occur because the prolonged intake of excessive iodine levels increases plasma iodine, which in turn has a negative effect on iodine uptake by the thyroid gland. In general, observations in dogs and cats are restricted to more subtle effects.

Dog:

When dogs were given a single dose of potassium iodate delivering 100 mg iodine/kg bodyweight (BW) it caused anorexia and occasional vomiting. Higher doses of 200 and 250 mg iodine/kg BW caused anorexia, coma and death. A lower (but still high) intake of iodine of 775 µg/kg BW/day in puppies was associated with bone abnormalities. This intake is equivalent to a dietary iodine content of about 17-18 mg/kg DM.

Cat:

There are conflicting reports of the effects of excessive iodine in cats. For example, Kyle et al. fed a diet containing an iodine content of 21 mg/kg DM to cats for 5 months and observed no clinical abnormalities or changes in serum free thyroxine levels. In contrast, another study reported a significant depression in serum free thyroxine when cats were fed a dietary iodine level of 13.8 mg/kg DM for only two weeks. Perhaps this was a temporary effect that eventually would have reverted to normal. It has also been suggested that excessive dietary iodine is a potential cause of feline hyperthyroidism but epidemiological studies have yet to identify a causal relationship.

Dietary Sources

Iodine is widely distributed in nature but in very small amounts and with large variations. The iodine content of crops depends on the iodine level in the soil. Soils particularly low in iodine are often designated as goitre regions where forages have to be supplemented with iodine to provide an adequate diet. Protein sources of marine origin are generally good sources and oilseed proteins and concentrates contain reasonable amounts. To ensure adequate dietary contents, dog and cat foods are usually supplemented with inorganic iodine salts such as potassium iodide and calcium iodate.