Foods high in copper can suppress thyroid activity. These include: avocadoes, liver, soybeans and walnuts. Foods or supplements high in calcium can suppress thyroid activity. Dairy products are not only high in calcium, but in fat, which can reduce thyroid activity. High calcium in the diet can lower potassium, a mineral needed to sensitize the tissues to thyroid hormones.
Goitrogenic foods (members of the cabbage family) contain thiocyanates which inhibit thyroid function. These include Chinese cabbage, broccoli, water cress, kale, rutabaga, turnips, brussels sprouts, cauliflower, radish, horseradish, collard, kohlrabi, rape and mustard greens. These foods should be avoided by individuals suffering from decreased thyroid activity.
A high fat diet tends to slow the thyroid. Fat tends to slow the metabolic rate. Dietary protein has a stimulatory effect on thyroid activity. A diet low in protein results in diminished thyroid activity. In some individuals, an inability to properly digest protein due to a zinc deficiency or a deficiency of hydrochloric acid in the stomach, can lead to lowered thyroid activity.
Certain amino acids may affect thyroid activity. L-tyrosine can help stimulate production of thyroxin. Whole grains are high in phytin. Phytin combines with calcium, magnesium and zinc, resulting in reduced absorption of these minerals. Lowered calcium and magnesium can temporarily stimulate thyroid activity. However, lower zinc and the serotonin-stimulating effect of grains actually reduce thyroid activity.
Foods that cause a calcium loss may enhance thyroid activity. These foods include citrus fruits, salt and all foods high in sodium.
"A tendency to retain water and occasionally also sodium chloride, is common in the hypothyroid, particularly in the obese type, while the lean type hypothyroid is more likely to show dry salt retention. Restriction of salt is advisable in both cases. Potassium is often beneficial." --Practical Endocrinology pp. 255
"What causes all this thyroid trouble? Well, a lot of the problem is in the way we eat. Many of us include salt in our diet, for example, in endocrinology (56:387, 1955), mice tested for goiter-activating foods showed an enlargement of the thyroid could be caused by salt. With omission of salt, goiters decreased in size..." --EOCD pp.760
Salty foods tend to lower magnesium, which enhances thyroid activity. Many people mistakenly avoid all salt in their diet. Adding back a reasonable amount of sea salt can often improve symptoms of hypothyroidism.
Alcohol causes a magnesium loss from the body. Since magnesium acts as a brake on thyroid activity, alcohol can cause a temporary increase in thyroid activity.
Hope this helps!!!