HORMONE HIRIT – Trick or Treat?
Patricia B. Gatbonton, MD, FPCp, FPSEM
My colleague on these pages, Dr. Nemencio Nicodemus, has been writing on thyroid disorders over the last two years, but I’d like to take this opportunity to elaborate about how thyroid hormone affects our bodies and briefly discuss thyroid hormone replacement specifically.
The thyroid gland is a butterfly shaped gland in the front of the neck (not to be mistaken for the Adam’s apple). It is the biggest, single organ/ factory specialized to produce hormones. The hypothalamus is a center in the brain which regulates thyroid function by secreting the releasing hormone, thyrotropin releasing hormone, TRH, to the anterior pituitary gland in the middle of the brain, which in turn releases thyroid stimulating hormone, TSH.
TSH regulates thyroid hormone synthesis and secretion by attaching to receptors on the thyroid cell which stimulates the gland to release the two thyroid hormones, levothyroxine (T4) and liothyronine (T3) which then exert their metabolic effect. Among their life – sustaining actions include promoting normal fetal and childhood growth and development; regulating heart rate and myocardial contractility; affecting gastrointestinal motility and renal water clearance; and modulating the body’s energy expenditure, heat generation, and weight maintenance.
How do hormones work? The simple analogy is that of a lock and key. Hormones are the keys that open the lock (the receptor on the cell wall or inside its nucleus) that then allows a complex reaction to take place and produce something else.
When the level of thyroid hormone in circulation is sufficient, the message is sent back to the hypothalamus and pituitary to reduce the release of releasing and secreting hormones-the negative feedback pathway.
When evaluating you for a goiter, aside from the history and the physical exam, the most important laboratory test your doctor will ask for is a blood level of TSH. The most common cause of high TSH level is a goiter with low thyroid hormone production (primary hypothyroidism). Weight gain, lethargy, slow heart rate, cold intolerance, abnormal menstruation, swollen and puffy face and legs, difficulty or delay in moving bowels (constipation) are the usual symptoms. We like to say the patient is “Iow batt,” compared to the “energizer bunny,” of hyperthyroidism.
Low TSH is due to excess thyroid hormone production and a goiter. If there is also protrusion of the eyeballs (exopthalmos), it is usually from an autoimmune Grave’s disease causing palpitations, weight loss, fatigue, frequent bowel movements, difficulty sleeping, emotional lability and fine finger tremors.
When the thyroid is unable to make enough hormone for the body’s use, for whatever reason, we need to augment or replace the hormone so that the body can continue to function normally. Synthetic T4 is now available in pure form, in multiple dose formulations, and is not expensive. There is no need to give T3 because it comes from T4 in peripheral tissues, so you get both even if you take only one tablet. It is preferable to prescribe T4 because the hormone can be given once a day, its half-life is approximately 7 days. Treatment effect is easy to monitor by following FT4 and serum TSH levels.
The daily dose of levothyroxine is age and dose related. The dose also depends on why you are getting the medication in the first place, whether for replacement or for suppression of TSH in patients with thyroid cancer. In very rare cases, for thyroid nodules for instance, you will only need to take it for a short period. Because of rapid turnover, infants and young children need a higher dose than adults. Elderly patients will need a lower dose. In most patients with hypothyroidism, your doctor will start you with the full estimated dose requirement immediately. After 4-6 weeks, your doctor will adjust the dose further based on the serum TSH level. The goal is to normalize the serum TSH, which is typically between 0.5 and 4 mU/L. If you are older, or if you have underlying heart disease, the physician will start you on a lower dose and increase it slowly while monitoring your clinical symptoms and TSH.
Some medications or conditions can affect the absorption of thyroid hormone in your stomach. Some antacids, aluminum hydroxide antacids, calcium, sucralfate or iron compounds decrease T4 absorption. In these patients, T4 should be given before breakfast, when the stomach is empty, and the other compounds taken 4 hours later, after lunch.
What patients need to understand is that you need to take levothyroxine for life. It is wrong to think that once you have the medication, you don’t have to see your physician any more. Regular TSH monitoring is necessary to make sure the level of thyroid hormone is sufficient because change in weight, age, pregnancy can all affect the need how much thyroid hormone you need at different times.
Once you are on thyroid hormone and have a normal TSH, you are EUTHYROID, and are back to normal thyroid status, and should be fine as long as you continue to take the medicine.
Circumstances in Which Levothyroxine Requirements May Be Altered*
Increased levothyroxine requirements
Mucosal diseases of the small bowel (for example, sprue)
After jejunoileal bypass and small-bowel resection
Pregnancy (35, 36)
Therapy with certain pharmacologic agents
Drugs that block absorption
Aluminum hydroxide (39)
Ferrous sulfate (40)
Possibly lovastatin (41)
Drugs that increase nondeiodinative T4 clearance
Possibly phenytoin (44)
Drugs that block T, to T, conversion
Amiodarone (45, 46)
Decreased levothyroxine requirements Aging (65 years and older) (47, 48)
• References arc given in parentheses.
Annals of Internal Medicine
Mandel, S. J. et. al. Ann Intern Med 1993;119:492-502
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