Selenium is a powerful antioxidant mineral that works synergistically with vitamin E to protect your cat's cells from oxidative damage. It is a key component of glutathione peroxidase enzymes, which neutralize harmful free radicals and peroxides that can damage cell membranes and DNA. Selenium is also essential for proper thyroid hormone metabolism — the enzymes that convert inactive thyroid hormone (T4) to active thyroid hormone (T3) require selenium. Given that hyperthyroidism is the most common endocrine disorder in older cats, selenium's role in thyroid health is particularly relevant. Selenium also supports immune function and reproductive health.
A small piece of cooked tuna (about 30g) provides roughly 20–30mcg of selenium — your adult cat needs approximately 15–30mcg per day, about the selenium content in a bite-sized piece of fish. Commercial cat foods provide adequate selenium, and supplementation should only be done under veterinary supervision due to selenium's narrow safety window.
0.0% of daily nutrient intake
Selenium makes up 0.0% of your cat's total daily nutritional requirements by weight. That's a tiny amount — but it matters.
Muscle weakness, impaired immune function, reproductive problems, and increased susceptibility to oxidative stress-related diseases. Selenium deficiency can mimic or worsen vitamin E deficiency, since both nutrients protect against oxidative damage.
Selenium has a narrow safety margin — the difference between adequate and toxic doses is smaller than for most minerals. Selenium toxicity (selenosis) causes loss of appetite, weight loss, hair loss, garlic-scented breath, liver damage, and neurological problems. Never supplement selenium without veterinary guidance.
| Life Stage | Size | Min | Max | Unit | Notes |
|---|---|---|---|---|---|
| Adult | — | 15 | 50 | mcg | Selenium has a narrow safety margin. Do not supplement beyond what is in a balanced commercial diet without veterinary guidance. |
| Juvenile | — | 10 | 40 | mcg | Kittens need adequate selenium for antioxidant protection during rapid growth. Quality kitten food provides appropriate levels. |
Source: NRC 2006, AAFCO 2024
Selenium is required for the enzymes that convert inactive thyroid hormone (T4) to active T3. Without adequate selenium, even sufficient iodine cannot support normal thyroid function.
What this means: Both iodine and selenium support thyroid health through different mechanisms. Fish is an ideal food source as it naturally provides both minerals. For cats with thyroid concerns, ensure both nutrients are within appropriate ranges.
Selenium and vitamin E are complementary antioxidants. Selenium-dependent glutathione peroxidase and vitamin E both protect cell membranes from oxidative damage, and adequate levels of each reduce the requirement for the other.
What this means: Ensure your cat's diet provides both selenium and vitamin E together. Fish-based foods naturally supply both. If supplementing one, consider whether the other is also adequate to maintain antioxidant balance.
