AAFCO requirements and physiological role
Per AAFCO 2024 Official Publication canine and feline nutrient profiles and NRC 2006 Nutrient Requirements of Dogs and Cats, iodine is one of the trace minerals (alongside iron, copper, zinc, manganese, selenium). The canine adult dry-matter minimum is 1.0 mg per kg, the feline adult minimum is 0.6 mg per kg, and growth/lactation minima are slightly elevated. AAFCO 2024 also assigns an upper limit for iodine: 11 mg per kg dry matter in canine adult formulations and 9 mg per kg in feline adult formulations — reflecting the narrow safety window between deficiency and excess.
Per Roti 1991 (Endocr Rev) thyroid hormone review, iodine is taken up from blood by the thyroid follicular cells via the sodium-iodide symporter (NIS), oxidized to elemental iodine by thyroid peroxidase (TPO), and covalently incorporated into tyrosine residues of thyroglobulin to form mono-iodotyrosine (MIT) and di-iodotyrosine (DIT) intermediates. Coupling of MIT + DIT produces T3 (3,5,3'-triiodothyronine), and coupling of two DIT produces T4 (3,5,3',5'-tetraiodothyronine, also called thyroxine). The hormones are stored in the colloid of thyroid follicles bound to thyroglobulin and released through proteolysis under TSH stimulation. Approximately 70–80 percent of body iodine is concentrated in the thyroid gland (which weighs less than 0.1 percent of body mass), reflecting the unusual tissue concentration ratio.
Pet food iodine sources and bioavailability
Per AAFCO 2024 ingredient definitions and NRC 2006, the principal pet food iodine sources are calcium iodate (Ca(IO3)2), the workhorse feed-grade source supplying both calcium and iodine, ethylenediamine dihydroiodide (EDDI), an organic iodine source historically used in cattle feed and some pet food, potassium iodide (KI), the most bioavailable inorganic source per Backer 2004 (Lancet) iodine pharmacology, and kelp (Laminaria digitata, Laminaria japonica, Ascophyllum nodosum) as a natural marine source supplying iodine alongside trace minerals (iron, manganese, magnesium) and modest macronutrients.
Per Backer 2004 (Lancet) and Hetzel 2002 (Lancet) iodine deficiency review, the bioavailability hierarchy is approximately: potassium iodide ~95 percent > calcium iodate ~85 percent > EDDI ~70 percent > kelp variable (50–90 percent depending on species and processing). The kelp variability reflects the natural iodine content range of 100–5,000 mg iodine per kg dried kelp, requiring batch testing for inclusion at controlled doses. Pet food formulations typically use calcium iodate as the bulk source supplemented with kelp at 0.1–0.3 percent inclusion when natural-marketing positioning is desired. The kelp source overlaps with our kelp explainer.
Feline hyperthyroidism and the Y/D iodine-restriction framework
Per Mooney 2014 (J Feline Med Surg) feline hyperthyroidism review and Peterson 2017 (J Feline Med Surg) hyperthyroidism management update, feline hyperthyroidism is the most common endocrinopathy of older cats, affecting approximately 10 percent of cats over 10 years of age. The condition involves benign or malignant hyperfunction of thyroid follicular cells producing excess T3 and T4, with clinical signs of weight loss despite increased appetite, polyuria/polydipsia, vomiting, and tachycardia. Conventional treatment options include radioactive iodine ablation (curative, gold standard per ACVIM 2016 guidelines), thyroidectomy (curative, surgical), and methimazole pharmacotherapy (suppressive, lifelong).
Per Hill’s Prescription Diet Y/D (Yellow Diet) framework introduced in 2011 and Mooney 2014 efficacy review, an alternative non-pharmacological approach restricts dietary iodine to less than 0.32 mg per kg dry matter, substantially below the AAFCO 2024 feline adult minimum of 0.6 mg per kg. The mechanism limits substrate availability for thyroid hormone synthesis, normalizing T4 levels in approximately 70–90 percent of treated cats per Mooney 2014. The diet must be fed exclusively (no treats, no other food) and is not a curative treatment — T4 levels rise within weeks of returning to a normal-iodine diet. The Y/D framework overlaps with our best cat food for hyperthyroidism guide. Per ACVIM 2016, Y/D is one of four accepted hyperthyroidism management approaches with discussion of pros/cons in the consensus statement.
Iodine deficiency and excess clinical signs
Per Roti 1991 (Endocr Rev) and Hetzel 2002 (Lancet) iodine deficiency review, dietary iodine deficiency in dogs and cats consuming AAFCO-complete commercial formulations is essentially unreported. Severe experimentally-induced iodine deficiency in dogs produces goiter (compensatory thyroid hyperplasia under TSH stimulation), hypothyroidism with associated lethargy and weight gain, and reproductive failure including stillbirth and neonatal cretinism. Spontaneous deficiency in companion animals is exceedingly rare given the iodine content of all commercial formulations.
Iodine excess can be more clinically relevant per AAFCO 2024 upper limit framework: prolonged intake exceeding 11 mg per kg dry matter (canine) or 9 mg per kg (feline) can paradoxically suppress thyroid function through the Wolff-Chaikoff effect (transient suppression of thyroid hormone synthesis under iodine excess) per Markou 2001 (Thyroid). Some early raw-fed and home-prepared diet recipes that included excessive kelp or iodized table salt produced inadvertent iodine toxicity in case-report literature; commercial formulations are validated for compliance with AAFCO upper limits. The hypothyroidism management framework overlaps with our best dog food for hypothyroidism guide.
How KibbleIQ scores iodine
The KibbleIQ Dry Kibble Rubric treats iodine as a neutral standard mineral. All commercial pet foods labeled "complete and balanced" per AAFCO 2024 supply iodine within the 1.0–11 mg per kg (canine) or 0.6–9 mg per kg (feline) range, so the presence of calcium iodate, EDDI, potassium iodide, or kelp in the ingredient list is expected and not a differentiating quality signal. The rubric does not award additional credit for kelp-sourced iodine over calcium iodate; the bioavailability ranking exists per Backer 2004 (Lancet) but does not warrant a rubric tier shift.
Cat foods explicitly marketed for hyperthyroidism management (Hill Y/D) use iodine restriction as a therapeutic feature distinct from the standard nutritional framework; the rubric does not separately score this since the diet operates outside the AAFCO complete-and-balanced framework as a therapeutic medical food. To check whether your dog’s or cat’s food contains iodine and what source, paste the ingredient list into the KibbleIQ analyzer. For peer mineral context, see our selenium explainer, magnesium explainer, zinc supplements explainer, and kelp explainer. For thyroid-related guides, see our best cat food for hyperthyroidism guide and best dog food for hypothyroidism guide. For methodology context, see our published methodology.