The Thyroid Gland

All animal life requires oxygen for sustenance, and the human species is no exception. Oxygen drives the basic metabolic processes that permit growth, development, reproduction, physical movement, and constant body temperature. The complex of chemical interactions necessary to sustain these processes is called metabolism, and the prime, overall regulators of metabolism are the thyroid hormones.

The thyroid gland is located in the anterior part of the neck in the midline. It consists of two lateral lobes lying on each side of the thyroid cartilage (Adam's apple) and connected by a band of tissue called the isthmus.

It is one of the larger endocrine glands, and its capacity to grow is phenomenal. Any enlargement of the thyroid, regardless of cause, is called a goitre. The thyroid arises in the embryo from a downward outpouching of the floor of the fetal pharynx, and a persisting remnant of this migration is known as a thyroglossal duct.

If viewed under a three-dimensional microscope, the resting thyroid is seen as a collection of small, generally globular sacs, called follicles, filled with the prohormone thyroglobulin. The cells lining these globules are called follicular cells, and it is their function to synthesize thyroid hormones as part of the prohormone thyroglobulin and either to secrete them directly into the circulation or store them within the follicles. When the individual's requirement for thyroid hormone increases, thyroglobulin is split into its component parts, and the thyroid hormone thus released passes through the follicular cells to enter the circulation. Nestled in the spaces between the follicles are parafollicular cells. These, in essence, form a separate endocrine organ. They have an entirely distinct embryological origin, and they are not embedded in the substance of the thyroid gland, in many species other than man (see {parathyroids} the parathyroid glands: calcitonin).

The thyroid hormones are not proteins; rather, they are modifications, called thyronines, of an amino acid, tyrosine. Thyroid hormones are heavily laden with iodine. The major active thyroid hormones are thyroxine (T4) and triiodothyronine (T3). Even though the thyroid gland manufactures considerably more T4 than T3, T3 is roughly 2 1/2 times more potent than T4. Indeed, in many ways, T4 serves as an additional, circulating depot for T3 in that when T4 leaves the circulation and travels through the cytoplasm to the nucleus of the target cell, its action at that site is preceded or accompanied by its conversion to T3.

Most of the T4 and T3 secreted by the thyroid is bound to special proteins (thyroxine-binding globulin [TBG] and prealbumins) in the serum, although small amounts of these hormones travel freely in the serum and are readily taken up by tissues to be replenished instantaneously from the T4 that had been attached to the binding proteins.

Essentially all the cells in the body are target cells of thyroid hormones. The major function of the thyroid hormones is to stimulate the synthesis of protein once they have entered the cell nucleus. Another important function is to stimulate the activity of the cell's mitochondria. These intracellular organelles are the sites at which there is a controlled exchange of energy. Some energy is conserved for the body's functionings, while the remainder is dissipated as heat. The proportion of energy devoted to each of these processes is controlled by the thyroid hormones. There are other intracellular thyroid hormone functions that are not well understood, but it is clear that thyroid hormones modulate protein, carbohydrate, fat, and vitamin metabolism, as well as the generation of body heat. Thyroid hormones also modify the activity of the autonomic nervous system.

While multiple factors, including nerves supplying the thyroid gland, influence thyroid hormone secretion, by far the major influences are the negative feedback loops. The thyroid is a prime example of the negative feedback effects of the hypothalamic-pituitary-target organ axis. Briefly, thyroid hormones inhibit the release of thyrotropin-releasing hormone (TRH) from the hypothalamus and thyrotropin (thyroid-stimulating hormone [TSH]) from the anterior pituitary. Increased consumption of thyroid hormones decreases their concentration in the circulating fluids, resulting in enhanced thyrotropin secretion and thus an increased thyroid hormone secretion until a normal serum level is regained. Conversely, with the administration of the thyroid hormones, the resultant increased serum levels inhibit TRH and thyrotropin secretion and reduce the secretion of thyroid hormone from the thyroid gland until the elevated circulating thyroid level is returned to normal. If an amount of thyroid hormone equal to the normal daily thyroid output is administered to a patient, the thyroid gland is effectively suppressed; it produces no thyroid hormone because levels of circulating TSH are greatly reduced.

There is an important extrathyroidal mechanism for modulating thyroid hormonal activity, that is, the controlled conversion of T4 into either the potent hormone T3 or the inactive molecule rT3. Tissue enzymes, particularly abundant in the liver and kidney, control the conversion of T4 to T3 or reverse triiodothyronine (rT3). Consequently, when T4 is metabolized to T3, thyroid hormone action is enhanced. Similarly, when the pathway for the conversion of T4 to rT3 is favoured, T3 levels fall and thyroid hormone activity in that particular tissue is proportionally decreased.

Aside from the regulatory functions, other factors, external or internal, may also influence the circulating levels and utilization of thyroid hormones. In all forms of malnutrition, including anorexia nervosa, there is a significant reduction in the conversion of T4 into T3. The commensurate decrease in oxygen consumption and metabolic rate has survival value for a person deprived of adequate food to sustain health; in effect, death from starvation is postponed. Iodine intake is important because an inadequate dietary supply leads to reduced circulating thyroid levels and an ensuing increase in serum thyrotropin levels. This increase, while perhaps not adequate to produce sufficient thyroid hormone, nevertheless stimulates growth of the thyroid, with the resultant appearance of a goitre. In the short term, low environmental temperature leads to increased utilization of thyroid hormones, activation of the hypothalamic-pituitary-thyroid axis, and a consequent rise in T4 and T3 levels. As the environmental temperature rises, the converse results, and small, appropriate changes in normal persons have been noted with changes of season.

Finally, thyroid hormone levels may be affected by many illnesses that have nothing directly to do with the thyroid gland. For this reason, it is not easy to ascertain with certainty the influence of aging on thyroid hormone activity because it is difficult to accumulate large numbers of aged subjects who can be said to be free of any disease. It is generally agreed, however, that few important changes occur in thyroid activity during the normal aging process.

The person secreting too little thyroxine and / or triiodothyronine can exhibit some of the following symptoms:	Fatigue Weakness Weight gain or increased difficulty losing weight Coarse, dry hair Dry, rough pale skin Hair loss Cold intolerance (can't tolerate the cold like those around you) Muscle cramps and frequent muscle aches Constipation Depression Irritability Memory loss Abnormal menstrual cycles Decreased libido
The person over-secreting thyroid hormones can exhibit some or all of the following:	Palpitations Heat intolerance Nervousness Insomnia Breathlessness Increased bowel movements Light or absent menstrual periods Fatigue Fast heart rate Trembling hands Weight loss Muscle weakness Warm moist skin Hair loss Staring gaze

1) Constant exposure to cold environments will cause the {hypothalamus} hypothalamus and {pituitary} pituitary to increase their thyroid hormone output.

2) Prolonged emotional stress can also affect TSH and thus cause thyroid hormone output to change. If the stress isn’t properly dealt with, thyroid dysfunction will eventually result.

3) Nerve pressure at the mid neck; see {chiro_subl} the spine) can cause thyroid dysfunction. It is not uncommon to see car accident victims gain 10-30 pounds with no other apparent reason.

4) The thyroid and adrenal glands work in concert. If a person suffers from hypoadrenia (see the {adrenals} adrenal glands), the thyroid will often suffer because they slow down to decrease your metabolic rate and give the adrenals a chance to rest.

5) The thyroid affects insulin secretion, and a person who has a history of prolonged consumption of refined carbohydrates and sugar can cause the thyroid gland and lead to dysfunction. The human body is not designed to handle large amounts of refined products and physiologically we pay the consequences when overindulging.

6) Birth control pills affect several hormones and due to the impact they create on epinephrine and cortisone levels, the thyroid can suffer as a result.

7) There are inherent risks in taking synthetic thyroid hormones for an inactive thyroid gland. See more about treating the thyroid {conditionhypothyroid} here.

8) Over consumption of fats and sugars over prolonged periods will cause a sluggish thyroid. The {hair_what_is_it} hair tissue mineral analysis can reveal imbalances that indicate the thyroid is over stressed.

9) X-rays (including dental x-rays) can damage the thyroid.

10) Prolonged intake of vitamin A supplements and also zinc can lead to hypothyroidism.

11) A deficiency of iodine is implicated in hypothyroidism.

12) Hormonal imbalances due to the {pituitary} pituitary, {liver} liver, {ovaries} ovary, or {adrenals} adrenal malfunction) can alter thyroid function.

Not all cases of thyroid dysfunction will show up on blood tests. An easy way to check your metabolic rate at home and thus get a good picture of thyroid function is by keeping a temperature chart. The Barnes Basal Temperature Chart Method is the preferred method to monitor the thyroid. Read about how to make your chart {thyroid_temp_regulation} here.

For a thorough discussion of the treatment of thyroid problems, go {conditionhypothyroid} here.

Like all other endocrine glands, various reflex points can be found to be involved when the thyroid is malfunctioning. The T1, T4 and T3 reflexes (not covered on our site), are found many times. Other CRA™ reflexes can also be active when the thyroid is involved:

System or Event Affected	Actions of T3/T4
Basal Metabolism	increases basal metabolic rate increases body temp (calorigenesis) increases appetite
Carbohydrate, lipid & protein Metabolism	promotes glucose catabolism for energy stimulates protein synthesis increases lipolysis enhances cholesterol excretion in bile
Heart	promotes normal cardiac function
Nervous System	promotes normal neuronal development in fetus and infant promotes normal neuronal function in adult enhances effects of sympathetic nervous system
Musculoskeletal	promotes normal body growth and maturation of skeleton promotes normal function and development of muscles
Reproductive	promotes normal female reproductive ability and lactation