Iodine is a trace mineral that your horse's body uses almost exclusively for one purpose: making thyroid hormones (T3 and T4). These hormones regulate metabolic rate, body temperature, growth, and reproductive function — they essentially set the speed at which every cell in the body operates. Pregnant mares need adequate iodine for proper fetal development, particularly of the foal's thyroid gland, brain, and musculoskeletal system. Iodine-deficient foals can be born with goiter (enlarged thyroid) or limb deformities. Regional soil iodine levels vary, and hay from iodine-poor regions may not provide enough. Iodized salt blocks are the simplest way to ensure your horse receives adequate iodine.
Your horse needs about 1 to 5 milligrams of iodine per day — less than a grain of sand. For a 500kg horse, the NRC recommends roughly 1.75mg for maintenance and up to 3.5mg for pregnant mares. An iodized salt block or loose iodized salt easily meets this need. Be cautious with kelp or seaweed supplements, as their iodine content can be unpredictable and may push intake into the toxic range.
0.0% of daily nutrient intake
Iodine makes up 0.0% of your horse's total daily nutritional requirements by weight. That's a tiny amount — but it matters.
Goiter (visibly enlarged thyroid glands on either side of the throat), dull or dry coat, lethargy, cold intolerance, poor reproductive performance, irregular heat cycles in mares, and weak or stillborn foals with goiter or contracted tendons. Mild deficiency may show as a subtle lack of energy or condition without obvious goiter.
Excess iodine is a genuine concern for horses. Paradoxically, iodine toxicity also causes goiter and can produce foals with the same thyroid enlargement and limb deformities as deficiency. Over-supplementation from kelp or seaweed supplements is the most common cause — some kelp products contain wildly variable iodine levels. The NRC upper safe limit is about 5 mg/kg of diet.
| Life Stage | Size | Min | Max | Unit | Notes |
|---|---|---|---|---|---|
| Adult | — | 1.75 | 5 | mg | For a 500kg horse. Provided by iodized salt blocks. The upper safe limit is approximately 5mg/kg diet — do not oversupplement with kelp or seaweed. |
| Pregnant / Nursing | — | 2.5 | 5 | mg | Pregnant mares need adequate iodine for fetal thyroid development. Both deficiency and excess cause goiter in foals. Maintain iodized salt access without over-supplementing. |
Source: NRC 2007
Goitrogens from cruciferous plants compete with iodine for thyroid uptake, effectively blocking thyroid hormone production. In a horse with marginal iodine status, even moderate goitrogen exposure could tip the balance toward hypothyroidism.
What this means: If you feed cruciferous vegetables (kale, broccoli, cauliflower) as regular treats, make sure your horse has consistent access to iodized salt. For pregnant mares, minimize cruciferous treat feeding and ensure iodine status is adequate.
