Low progesterone is often misdiagnosed as thyroid deficiency. Nevertheless, Dr. Peat acknowledges that thyroid hormone is basic to all biological functions and that sometimes both thyroid and progesterone supplements are needed, as "each has a promoting action on the other." To see whether thyroid supplementation might be needed in addition to the progesterone, he recommends a test called the Achilles tendon reflex, which measures muscle energy by the speed at which the calf muscle relaxes.

"Without adequate thyroid," says Dr. Peat, "we become sluggish, clumsy, cold, anemic, and subject to infections, heart disease, headaches, cancer, and many other diseases and seem to be prematurely aged.... Foods aren’t assimilated well, so even on a seemingly adequate diet there is ‘internal malnutrition.’ Irregular periods, often leading to needless hysterectomies, are common aspects of hypothyroidism; and breast disease, he says, is another classic manifestation. In explaining this, Mark Perloe, M.D., says, "Too little thyroid production may cause... increased prolactin levels and persistent estrogen stimulation."

In a conversation, Dr. Peat told us that estrogen (which we can try to balance with supplemental progesterone) inhibits the release of thyroid hormone from the gland, whereas an adequate amount of thyroid hormone, on the other hand, raises natural progesterone production and lowers estrogen. That makes it easy to see how thyroid hormone and progesterone can complement each other. He even made the interesting observation that since estrogen and cortisone weaken the blood vessels, progesterone (along with thyroid supplements) is a good way to help prevent easy bruising.