Just below your Adam's apple sits the butterfly-shaped thyroid gland. The thyroid secretes two hormones - and these hormones have everything to do with your metabolism (the conversion of nutrients into energy); body temperature; heart rate and blood pressure; bowel function; skin, hair, and nail growth; cholesterol levels; and numerous other functions.

An underactive thyroid fails to produce enough of the hormones it is supposed to produce. This condition is called hypothyroidism (it affects women more than men). An overactive thyroid produces too many hormones - this condition is called hyperthyroidism. Since an underactive thyroid is the most prevalent, we will limit this discussion to hypothyroidism.

There are several causes of hypothyroidism - but whatever the cause, the end result is the same: A slow metabolism which affects every single cell in your body. This leads to weight gain as well as feeling tired all of the time. Constant fatigue also affects your brain, leaving it too tired to motivate your body into action. To other people, you may appear lazy. Is it any wonder that hypothyroidism often leads to depression?

Besides weight gain, fatigue, and depression - there are other symptoms of hypothyroidism. Poor digestion, constipation, feeling cold while those around you are warm (a particularly miserable symptom if you're going through menopause and having hot flashes too), aching muscles and joints, dry skin and brittle nails, thinning hair, irregular menstrual cycles, high cholesterol, and recurring infections are all potential symptoms of hypothyroidism.

Four common reasons for hypothyroidism are (1) not enough of the mineral selenium, (2) a damaged thyroid gland, (3) miscommunication between the brain and the thyroid about hormone production, and (4) not enough of the mineral iodine.

Let's take a look at each dysfunction.

(1) The thyroid produces two hormones: T3 (which contains 3 iodine molecules) and T4 (which contains 4 iodine molecules). T3 is more active and about four times more potent than T4, and on an as-needed basis the body converts T4 to T3. Since roughly 80% of your thyroid hormones are T4 and 20% are T3, there is plenty of T4 hanging around to be converted to T3.

Conversion takes place mostly in the thyroid gland and liver, and conversion requires a special enzyme to remove one iodine from T4 to make it into T3. That enzyme can only do its job when the mineral - selenium - is available to 'activate' the enzyme. Too little selenium, and the conversion of T4 to T3 is sluggish, slowing down your metabolism.

(2) Sometimes the body attacks its own thyroid gland through a (still) mysterious autoimmune disease mechanism. This apparently happens when the immune system creates an antibody against something else, but the antibody 'cross-reacts' and attacks the thyroid gland.

Researchers think food allergies could be involved in producing this antibody; or, during pregnancy, cells from the fetus could be seen as foreign to the body, thus producing the antibody (helping to explain why hypothyroidism is mostly found in women).

(3) The thyroid gland is controlled by the pituitary gland - a peanut-sized gland located just under the brain. The pituitary gland is controlled by the hypothalamus - a part of the brain.

Here's how it works: When the brain detects low levels of T3 and T4 in the blood, it sends a hormone (TRH) to the pituitary gland. The pituitary gland then sends a hormone (TSH) to the thyroid that relates this message: Make more T3 and T4.

Any disease state involving the brain or pituitary gland could lead to a miscommunication with the thyroid about hormone levels. Most often, it is the pituitary gland that has a problem.

(4) A low iodine supply causes a problem for the thyroid in producing the (3-iodine) T3 hormone and (4-iodine) T4 hormone. This is not 'much' of a problem in the U.S. anymore because we use iodized salt. However, if you have all but given up salt (usually to control high blood pressure), you may be low in iodine.

T3, T4, antibodies that attack the thyroid, TRH, TSH, and iodine can all be measured with lab tests to determine if you have hypothyroidism. These tests are reasonably accurate, but sometimes miss low or moderately-low hypothyroid conditions.

If you have any of the symptoms listed above for hypothyroidism, get the lab tests done.

In Part 2, I'll say a bit more about thyroid tests and share some tips for supporting your thyroid function.