Thyroid Hormones and Function—An Overview
by Rashmi Gulati, MD
Providing the body with extraordinary metabolic activity, our thyroid hormones participate in pathways that regulate energy consumption, protein synthesis, calcium balance, and hormone responsiveness. Maintained in delicate balance through a set of complex feedback loops, these hormones ultimately define the rate and efficiency of many of our metabolic processes, and indirectly but powerfully influence the way we feel physically, mentally, and emotionally.
Because of the thyroid hormones' widespread effects, thyroid dysregulation can significantly affect a person's life and poses a detriment to long-term health. Here's an overview of how the thyroid complex works when an individual is euthyroid, meaning their thyroid function is "well" or "good."
Weighing in at just about an ounce on average, the thyroid is positioned at the front of the neck, just below the Adam's apple. This butterfly-shaped endocrine gland is made up of many lobules that house spherical, or sac-like follicles where thyroid hormones are synthesized. Thyroid hormones are secreted from the follicles' exterior surfaces, predominantly as a molecule known as thyroxine, or T4, but with nominal amounts of triiodothyronine (T3), and reverse T3 (rT3) being produced and secreted there as well.
A fourth hormone, calcitonin, is secreted from a different type of cell interspersed between and amongst the follicular thyroid cells, called the parafollicular cells. In addition, there is another component to the thyroid–endocrine complex called the parathyroid glands, which comprise four smaller, functionally and anatomically discrete "buttons" of tissue embedded posteriorly in each of the thyroid gland's "butterfly wings." These four parathyroid glands secrete parathyroid hormone (PTH).
Of the thyroid hormones, T4 is the only one that appears to arise exclusively from the thyroid. Structurally, the principal thyroid hormones T4 and T3 differ only by a single iodine atom: T4, the weaker prohormone, or precursor to T3, is produced in far higher amounts and converted peripherally in the body to the more active T3 form, through loss of the fourth iodine atom. T4 circulates throughout the body to be converted to T3 as needed at receptor sites in diverse tissues, primarily in the liver and kidneys, but also in nervous system tissues, as well as by the thyroid. This conversion reaction requires the mineral selenium.
T3 activates the production of proteins involved in the breakdown of nutrients to produce energy and heat. Through these pathways, T3 and T4 regulate metabolism, body temperature, growth, and heart rate, among numerous affiliated and ancillary tasks. The iodide released into the blood in this process can be recycled. The other thyroid hormone, calcitonin, contributes more specifically to the maintenance of physiologic blood calcium levels and bone mineral metabolism.
Release of thyroid hormones T4 and T3 is predominantly controlled by the hypothalamus, the portion of the brain considered to be the "master regulator" of hunger, thirst, fatigue, circadian rhythms, and many other complex processes, including the life-saving stress response. If a shift in one of these physiological states is warranted, the hypothalamus releases thyrotropin-releasing hormone (TRH), which travels to the pituitary, issuing the edict to produce thyroid-stimulating hormone (TSH), which in turn communicates with the thyroid gland to stimulate thyroid hormone secretion. This represents the hypothalamic–pituitary–thyroid or HPT axis. Beyond this, T4 and T3 levels are balanced through a negative feedback loop with thyrotropes, the cells in the pituitary gland that release TSH, and to a lesser extent via the hypothalamus.
In this healthy system, at a point where T4/T3 levels reach sufficiently high levels, thyroid-stimulating hormone production will taper off, and production of additional thyroid hormones will also taper as a natural consequence. If T4/T3 levels become low, increased TSH output from the pituitary stimulates their production by the thyroid, to maintain a minute-to-minute, dynamic physiologic balance. Though primarily controlled via the HPT axis, some of this thyroid gland activity is autoregulated through intrathyroid pathways, as well as peripherally, through the peripheral synthesis and action of thyroid hormones at target cells throughout the body.
Calcitonin production in the parafollicle cells is stimulated through a different mechanism: in response to elevated blood levels of calcium. Calcitonin secretion is suppressed in turn when calcium blood levels fall below normal. Calcitonin's major role is to prevent blood calcium levels from becoming too high, a condition known as hypercalcemia.
The parathyroid glands are another subcomponent of the thyroid gland complex involved in calcium homeostasis. Should blood calcium become too low, calcium-sensing receptors in the parathyroid glands stimulate the release of parathyroid hormone (PTH) which, along with calcitonin, helps to rebalance levels.
Calcium levels in the blood are maintained within a tight range with the aid of these two subunits of the thyroid gland complex. Generally, PTH exerts greater control of calcium metabolism than calcitonin, so removal of the parathyroid glands can have a more profound effect on blood calcium balance than thyroid removal.
Thyroid hormones interact in many significant ways with other hormones in the body. Though the intricacies of these interactions are not entirely understood, we do know that high levels of stress hormones, in particular, can produce "noise" that has a disruptive effect on pathways involving thyroid hormones. As all our hormones are engaged in a ongoing balancing act, chronic high stress, which amplifies activity in the hypothalamic–pituitary–adrenal (HPA) axis, is clearly associated with dysregulation of the related HPT axis. There is also evidence that thyroid hormones regulate the expression of sex hormone receptors, to modulate the effects of estrogen, prolactin, and progesterone in the body.
Supporting Your Hormonal Health and Well-Being
A well-functioning thyroid is essential to your overall health. If you feel that your thyroid is not functioning properly, there are measures you can take to minimize the health risks and damage that thyroid dysregulation may pose. We have extensive experience in treating thyroid hormone imbalance, thyroid diseases, and other endocrine disorders. With a range of tests offering greater specificity and sensitivity than conventional testing, the expert physicians at Patients Medical can provide timely and accurate diagnosis. In partnership with you, we design and coordinate an individualized integrative protocol, combining the best of conventional and alternative evaluation and treatment solutions to help you achieve maximum health.
For those that can make the journey, we are happy to welcome new patients to our medical center
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Citation and Further Reading:
Varas, S., & Jahn, G. 2005. The expression of estrogen, prolactin, and progesterone receptors in mammary gland and liver of female rates during pregnancy and early postpartum: Regulation by thyroid hormones. Endocr. Res., 31 (4), 357–370. URL (abstract): http://www.ncbi.nlm.nih.gov/pubmed/16433254 (accessed 04.20.2010).
Date of Publication: 09/05/2005
Article Last Updated: 11/17/2011