Thyroid hormones consists of either levothyroxine (T4) or triidothyronine (T3) which are produced in the thyroid gland. The thyroid gland is a butterfly shaped gland located in the front of the neck. T4 is produced in the thyroid gland while the majority of T3 is produced at the cells by conversion of T4 to T3. T4 is the storage form of thyroid while T3 is the activated form of thyroid. It is best to measure levels of free T4 and free T3 which measures only that amount of hormone that is freely available to the tissues. TSH is the pituitary hormone, which regulates the hormone production by the thyroid gland.
The actions of thyroid hormone are to regulate the rate of metabolism and affect the growth and rate of function of many other systems in the body. Thyroid hormones are essential to the proper development and differentiation of all cells of the human body. These hormones also regulate protein, fat, and carbohydrate metabolism and how human cells use energy. Thyroid hormone is essential to brain development and without thyroid a newborn baby will develop cretinism a severe form of mental retardation with stunted growth. All newborn babies are tested for low thyroid.
The most frequently prescribed thyroid hormones are levothyroxine (Synthroid, Levoxyl, Levothroid), triiodothyronine (Cytomel) and Armour Thyroid, Armour is a preparation made from the thyroid glands of cows and pigs and it contains both T4 and T3. Some patients do better with a combination of T4 and T3. Thyrolar is a combination of T4 and T3. Compounding pharmacists can also make up T3SR which is a sustained release form of T3 which may lessen the palpitations that can occur with Cytomel. Armour Thyroid 65 mg is approximately equivalent to 100 mcg of Synthroid and 25 mcg of Cytomel.
Hypothyroidism is probably the most commonly recognized endocrine disorder. Yet a century ago and even in the early 1900's mental hospitals were filled with patients with undiagnosed hypothyroidism because hypothyroidism can present with symptoms of mental illness such as memory loss, mental impairment and depression. All those patients went undiagnosed because the technology for measuring hormones was not developed until the 1960's. In fact the specialty of endocrinology did not even exist a century ago.
Hormones are present in the blood in very small amounts. For example, a normal estradiol level we aim for is 5 to 8 nanograms per milliliter. This is 5 to 8 billionths of a gram! For some endocrine tests, the levels are measured in trillionths of a gram. To measure such small amounts requires very sophisticated laboratory testing such as radioimmunoassay and immunofluorescence techniques. In comparison, most standard lab tests measure hundredths of a gram or thousands of a gram.
Hypothyroidism is most often caused by Hashimoto's thyroiditis, an autoimmune disease. It can also occur following surgical removal of the thyroid, following thyroid ablation with radioactive iodine and after external radiation to the head and neck. It can also occur from certain drugs such as lithium or interferon. Replacing thyroid to normal levels should result in improvement of most symptoms.
Hyperthyroidism is a condition wherein the thyroid gland overproduces thyroid hormone. This condition may be caused by a diffuse enlargement of the thyroid gland called Graves' disease or by a single nodule in the thyroid, which overproduces thyroid hormone. The symptoms include unexplained weight loss in spite of increased appetite, rapid heart beat which in older adults can cause dangerous heart arrhythmia, bulging eyes, heat intolerance with increased sweats, frequent bowel movements, tremors, fatigue and chronic insomnia. Hyperthyroidism can also be caused by taking excessive doses of thyroid hormone. Hyperthyroidism can be treated with anti-thyroid medications, radioactive iodine or with surgery.
Read Dr. Johnson's Feature Article on Hypothyroidism