Selenium is required by the body for proper functioning of the thyroid gland, and may help protect against free radical damage and cancer. A deficiency in selenium can lead to pain in the muscles and joints, unhealthy hair, and white spots on the fingernails. In long term cases it may even lead to Hashimoto's disease, a condition in which the body's own immune system attacks the thyroid. An excess of selenium can lead to bad breath, diarrhea, and even hair loss. The current daily value (DV) for selenium is 70μg (micrograms). It is important to note that the amount of selenium in any product varies greatly by the amount of selenium in the soil in which it was produced/grown/raised. Be sure to check individual labels, and if you have a deficiency in selenium, get tested after changing your diet to be sure you are eating adequate amounts. Below is list of high selenium foods by common serving size, for more see the list of high selenium foods by nutrient density, and the extended list of selenium rich foods.

#1: Brazil Nuts
Selenium in 100gPer Cup (133g)Per Ounce (28g)
1917.0μg (2739% DV)2549.6μg (3642% DV)536.8μg (767% DV)
Selenium Mixed Nuts (14%), Cashews (8%), Black Walnuts (7%), and Macadamia Nuts (5%). Click to see complete nutrition facts.

#2 Chicken and Turkey 
Selenium in 100gPer Cup Chopped (140g)Per 3 oz (85g)
37.8μg (54% DV)52.9μg (76% DV)32.1μg (46% DV)
Chicken is also High in Selenium Providing (%DV per 3 oz cooked): Roast Chicken Breast (39%), Chicken Thigh (36%), and Stewing Chicken (35%). Click to see complete nutrition facts.

#3 Seeds (Sunflower)
Selenium in 100gPer Cup (128g)Per Ounce (28g)
79.3μg (113% DV)101.5μg (145% DV)22.2μg (32% DV)
Other Seeds High in Selenium (%DV per ounce): Chia Seeds (22%), Sesame Seeds (14%), Flaxseeds (10%), and Pumpkin and Squash Seeds (4%). Click to see complete nutrition facts.

#4  Pork (Lean Tenderloin - Cooked)
Selenium in 100gPer 3 oz (85g)Per Chop (73g)
51.6μg (74% DV)43.9μg (63% DV)37.7μg (54% DV)
Pork Ham (61%), Lean Pork Mince (60%), and Lean Pork Loin (59%). Click to see complete nutrition facts.