Their (polyunsaturated oils) best understood effect is their interference with the function of the thyroid gland. Unsaturated oils block thyroid hormone secretion, its movement in the circulatory system, and the response of tissues to the hormone. When the thyroid hormone is deficient, the body is generally exposed to increased levels of estrogen. The thyroid hormone is essential for making the ‘protective hormones’ progesterone and pregnenolone, so these hormones are lowered when anything interferes with the function of the thyroid. The thyroid hormone is required for using and eliminating cholesterol, so cholesterol is likely to be raised by anything that blocks the thyroid function.
Mothers who have the autoimmune forms of thyroid disease will usually have thyroid autoantibodies present in their blood. Hypothyroidism is most commonly caused by Hashimoto’s Thyroiditis, an autoimmune thyroid disease in which the immune system attacks and destroys the thyroid gland, resulting in underproduction of thyroid hormone. Another common autoimmune thyroid disease is Grave’s Disease, which is a leading cause of hyperthyroidism; in this disease the antibodies attacking the thyroid gland cause growth of the gland and overproduction of thyroid hormone. Some mothers may be worried that these antibodies may pass into breastmilk and harm baby, however this is not a concern. The thyroid autoantibodies are IgG immunoglobulins, which are too large to pass into breastmilk. Continuing to breastfeed will only benefit your baby, as babies who are artificially fed are at increased risk of developing autoimmune thyroid disease themselves.