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Iodine - I in dog and cat diets

A short guide to the role of Iodine - I in dog and cat nutrition.

Iodine - I

It is an essential building block of the thyroid hormones (thyroxine T4 and triiodothyronine T3), which regulate the rate of metabolism, thermoregulation, heart and nervous system function, and also determine the proper growth and development of puppies and kittens [1]. It is one of the few minerals that, in a raw diet of land meat, is very likely to be too low — it requires deliberate supplementation. The NRC standard for an adult dog is MR 175 µg, RA 220 µg / 1000 kcal ME (upper limit SUL 2750 µg) [1]. For an adult cat the NRC gives higher values — MR 320 µg, RA 350 µg / 1000 kcal ME — although some newer studies suggest that the cat's actual requirement may be lower than the 2006 standard [2].

Where it's found: The meat and bones of land animals are extremely poor in iodine. In practice it is supplied from standardised marine algae (kelp, e.g. powdered Ascophyllum nodosum) with a known iodine content per gram. Random dried kelp "by eye" is a lottery — differences between batches can be as much as tenfold.

Deficiency: In animals fed commercial foods it practically never happens. It is, however, a common problem in unbalanced home diets. It results in hypothyroidism (the animal becomes apathetic, seeks warm spots, gains weight quickly), dermatological problems (dry, brittle and dull coat) and goitre (thyroid enlargement on the neck) [1] [3]. In puppies and kittens — delayed development, especially of the nervous system. Symptoms appear late, which is why iodine is not assessed by the animal's appearance but calculated.

Excess: The margin between an optimal and an excessive dose is narrower than for most minerals — don't double the dose "just in case". Chronic excess (most often from uncontrolled supplementation) paradoxically also inhibits the thyroid, giving symptoms resembling a deficiency [4]. In the cat, excess is particularly risky: low body weight makes overdosing kelp easy, and hyperthyroidism is the most common endocrinopathy of older cats — an iodine-restricted diet can normalise T4 levels in a significant proportion of affected cats [5].

What to watch for

Selenium is essential for the conversion of T4 into active T3 (selenium-dependent deiodinases) — with a selenium deficiency even a proper iodine supply is not enough [1]. In the cat a steady, daily dose is key — fluctuations in supply (once nothing, once a large portion of fish or kelp) are worse than moderate but stable supplementation. Watch out for tracheas, oesophagi and meat from the neck area — they may contain remnants of the slaughtered animal's thyroid gland, which means administering ready-made T3/T4 hormones, not iodine. This is a separate, serious problem (so-called dietary hyperthyroidism), particularly dangerous in the cat.

Sources

  1. National Research Council (2006). Nutrient Requirements of Dogs and Cats. National Academies Press. https://nap.nationalacademies.org/catalog/10668/nutrient-requirements-of-dogs-and-cats
  2. Wedekind KJ, et al. (2010). The feline iodine requirement is lower than the 2006 NRC recommended allowance. https://pubmed.ncbi.nlm.nih.gov/19906136/
  3. Merck Veterinary Manual. Goiter in Animals. https://www.merckvetmanual.com/endocrine-system/the-thyroid-gland/goiter-in-animals
  4. Feline Hyperthyroidism: Potential Relationship with Iodine Supplement Requirements of Commercial Cat Foods (2024). https://pmc.ncbi.nlm.nih.gov/articles/PMC11149000/
  5. van der Kooij M, et al. (2014). Effect of Feeding an Iodine-Restricted Diet in Cats with Spontaneous Hyperthyroidism. https://pmc.ncbi.nlm.nih.gov/articles/PMC4895365/