The thyroid gland consists of a small (several millimeters), bilobulated structural tissue partially surrounding an area of the primary airway in the neck (the trachea). (Sometimes, functional thyroidal tissue is also found in other locations.) This gland synthesizes the hormone thyroxine which, itself, is converted to other, active or inactive hormones. The synthesis and metabolism of thyroxine, its function and the functions of its metabolites in general, the regulation of its synthesis and activity...all this will be discussed below. Following this discussion of the basics of thyroid hormones and the effects of non-thyroidal illnesses on circulating hormone measurements, the reader should focus on the clinically important thyroid syndromes in dogs (hypothyroidism) and cats (hyperthyroidism).
The thyroid hormones affect virtually every metabolic activity in the body. These include the concentration and functionality of numerous enzymes, all aspects of the metabolism of fats, carbohydrates, protein, vitamins, the utility of minerals, the secretion and breakdown of all other hormones, as well as the response of tissues to these other hormones. The thyroid hormones have marked influence on the contractile strength and rhythm of the heart, they enhance respiratory drive especially when tissue/blood oxygen levels are suboptimal (i.e. promotes breathing!), they stimulate the marrow to create more red blood cells, if needed (erythropoiesis), and are active in the regulation of bone synthesis and turnover. They are important for processes where cell turnover and resynthesis are ongoing (e.g. the hair follicle). According to Feldman and Nelson* “...no tissue or organ system escapes the adverse effects of thyroid hormone excess or insufficiency.”
These consist of thyroxine, (also called T4 ), 3,5,3’-triiodo L-thyronine (also known as T3) and 3,3’,5’-triiodo L-thyronine (known as “reverse” T3 ...or rT3 ). All of these contain iodine bound to one or more carbons of complex rings that comprise the backbone structure of the hormones. The structure of each of these is shown in the diagram.
During metabolism, T4 is converted to T3 or to rT3 via removal of an iodine atom from one of the hormonal rings. The T3 is the biologically active thyroid hormone, whereas rT3 has no biological activity. It is presumed that factors that regulate whether T4 is converted to T3 or rT3 are pivotal in determining and thus regulating the functional activity of the thyroid at the level of the gland, itself. As we shall see, control and regulation of thyroid hormone activity is also accomplished at other levels.
It should be noted that the primary hormone created in the thyroid is T4 ..and that only about 20% of the T3 produced from T4 is produced in the thyroid gland. Instead, circulating free T4 is converted to T3 at the level of the target tissues...when hormone diffuses into cells or binds to tissue-specific receptors. T3 enters the cell, where is asserts its influence on function at the cytological or nuclear level, accordingly. It is important to add that there are other pathways of metabolism of thyroid hormones in target tissues not involving iodine removal or addition...and that these are essential to the functional influence of the thyroid upon those specific tissues.
One more thing that will be important, when we discuss clinical diseases and testing of thyroid function is the form of circulating T4 . Most...nearly all (99.9%)... of T4 in the circulation is bound to protein. Barely ~0.1% of T4 is free. Only the free T4 can be converted to T3 at the target tissue; however as free T4 is consumed, in principle more is released from protein to replace it. However, this process can be affected by other complicating factors (see non-thyroidal illness) that, also, will be discussed under clinical diseases and testing methods.
Regulation of thyroid hormones occurs from regions external to the gland and from within the thyroid itself.
In the brain there exists an important system-wide regulatory center known as the hypothalamus. The hypothalamus synthesizes and releases a hormone called thyrotropin-releasing-hormone (or TRH). The release of TRH by the hypothalamus causes release of thyrotropin-stimulating-hormone (or TSH) by the nearby pituitary gland. TSH from the pituitary travels via circulation to the thyroid tissues and is the most significant factor for stimulating the synthesis and release of biologically active thyroid hormone molecules.
The regulation of TRH synthesis and release from the hypothalamus is incompletely understood, but probably involves complex neurological signals and interactions that establish the initial “set-point” for TRH release. However, the hypothalamus is also sensitive to levels of TSH; when TSH levels are increased, there is feedback of this information to the hypothalamus resulting in the cessation of further synthesis and release of more TRH (this process is called feedback inhibition). It is also believed that the presence of adequate circulating levels of free T4 , and T3 also affect a feedback inhibition on the hypothalamus to cease TRH release AND on the pituitary to cease TSH release. A diagram illustrates these interactions..the lines in the image from the thyroid hormones reflect back to illustrate that thyroid hormones inhibit further stimulation of their own synthesis by the hypothalamus and the pituitary glands (dashed [----] lines and minus [-] signs).
The thyroid gland itself is able to affect some regulation of its activity ("autoregulation"). This can occur via controlling its uptake of iodine (essential to the synthesis and function of thyroid hormones), controlling its sensitivity to the influence of TSH, failure to synthesize adequate T4 when iodine availability is reduced, reducing T4 in the presence of excessive iodine, and possibly synthesizing rT3 in preference to T3 when some conditions warrant less biological activity.
The conditions which instigate and promote this thyroid gland autoregulation are poorly understood and probably more complicated than necessary for this discussion. They are only mentioned so that the reader is aware they exist.
This portion of the discussion will lead to first principles pertaining to how we test and interpret tests for aberrant thyroid hormone activity in dogs and cats . This will become relevant and important when the reader reviews the pages on Hypothyroidism (dogs) and on Hyperthyroidism (cats). The details of actual tests are found near the end of this page
Evaluation of thyroid gland hormone output (hypothyroid = not enough output; hyperthyroid = excessive output) can be accomplished via measurements of circulating thyroid hormone levels. The simplest and easiest to measure is total T4 (i.e bound to protein). It is also possible to measure freeT4 , T3 , and rT3 . Sometimes, it is useful to measure stimulating hormone levels (TRH or TSH) or the response of the thyroid gland to administration of one or the other of these stimulating hormones (TRH-Stimulation Test; TSH Stimulation Test) or to the administration of T3 on the level of T4 (T3 -Suppression Test). Details about these tests are discussed below. The specific indication for one or the other complex tests of thyroid function will be elaborated upon in the discussions of hypo- and hyperthyroidism.
Sometimes measured levels of circulating hormones are altered by illnesses (or recent anesthesia, Journal of Veterinary Internal Medicine January 2009) NOT related to the thyroid gland. In many non-thyroidal illnesses, total T4 measurements reveal lower than normal values; T3 may be anything (T3 alone is never reliable for evaluation of thyroid function), and freeT4 may be low, normal or high....but the animal is not truly hypothyroid OR hyperthyroid.
Causes for lowered T4 with non-thyroidal illnesses include:
Incidences of lowered T4 due to non-thyroidal illness (false positives):**
Causes for Normal or Elevated T4 when true hypothyroidism is present
Free T4 may also be artefactually altered with non-thyroidal illnesses
Incidences of lowered free T4 by equilibrium dialysis due to non-thyroidal illness (false positives):**
Reverse T3 (rT3 )...may increase with non-thyroidal illnesses....probably because there is decreased enzyme-mediated conversion of T4 to T3 , and the alternative pathway (to rT3 ) predominates...this is an avenue of research that could be helpful in distinguishing true thyroid pathology from artefacts due to non-thyroidal illnesses.
Testing for thyroid function is not an exact science and is, as discussed earlier, confounded when there is concurrent non-thyroidal illness. In both dogs and cats, several tests have been examined for efficacy, i.e. sensitivity and specificity. These tests include the following: Total T4 , freeT4 (by equilibrium dialysis), T3 Suppression Test, TSH Stimulation Test, TRH Stimulation Test, Serum TSH Concentration, Serum TRH Concentration, Serum T3 , Serum Reverse T3 (rT3 ).
*Feldman & Nelson, Canine and Feline Endocrinology & Reproduction, 2nd ed, Saunders 1996
**VIN and Intervet® Sponsored Symposium on Canine Hypothyroidism 6/26/06
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