This is because thyroid medication may cause the thyroid gland to stop producing thyroxin on its own. Also, some people have an auto-immune problem where their own body is attacking the thyroid gland. It usually does this as a result of a viral or parasitic infection in the gland. Another factor involved in thyroid function is diet. Diets low in protein and high in fats can interfere with thyroid function.
Adequate protein is required to produce the thyroid hormone.
Fat has been found to be antagonistic to thyroid function and it also lowers metabolism. Also, thiocyanate, a substance found in vegetables from the cabbage family, has an antagonistic effect on the thyroid. Excessive intake of thiocyanate foods can cause a disease called goiter. Goiter is an enlargement of the thyroid gland where it becomes slow and underactive. The solution to all this is to be tested by a qualified nutritionist in order to detect any vitamin or mineral imbalances.