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IntroductionThe multibillion dollar pet food industry has recently seen the addition of a new dietary therapy that promises to promote “thyroid health” in a can, or if you prefer, a bag. Despite the multimillion dollar advertising campaign waged by the Colgate-Palmolive subsidiary Hill’s Pet Nutrition that promotes an iodine deficient diet as a means of achieving “Thyroid Health”, thoughtful evaluation of their product yields serious concerns. |
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Thyroid Anatomy/PhysiologyThe thyroid is a small bi-lobed gland located in the cervical region (i.e., neck) in all mammals. The thyroid gland gets its name from the adjacent thyroid cartilage, often referred to as the “Adam’s Apple”, which is the largest cartilage of the larynx or voice box. The thyroid cartilage resembles an ancient Greek army shield called a thyreos which, like the cartilage, also had a notch at the top for the soldier’s chin [1] (Figure 1). The thyroid is one of the body's endocrine glands. Endocrine glands get their name from their behavior of secreting hormones into (i.e., “endo”) the blood stream. The hormones produced by the endocrine glands circulate within the bloodstream throughout the body to regulate various bodily functions. 
Figure 1. Illustration of the human and feline cervical regions with emphasis on the thyroid gland, a small bi-lobed gland located in the neck in all mammals that gets its name from the adjacent thyroid cartilage. In cats the central isthmus that connects the two lobes is often vestigial resulting in the presence of two anatomically independent lobes.
The thyroid gland consists of two cell types that differ based on their location and function (Figure 2). The majority of the cells within the thyroid gland are called follicular cells because they form numerous, microscopic, spherical structures called follicles. The follicular cells are responsible for the production of the thyroid hormones T3 (or triiodothyronine) and T4 (or thyroxine). The minority of cells in the thyroid are called parafollicular cells because they do not participate in the formation of thyroid follicles. The parafollicular cells produce a hormone called calcitonin that plays a relatively small role in calcium balance in the body [1]. |
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The follicles formed by the follicular cells surround a central cavity containing a material called the colloid which is rich in a hormone precursor, produced by the follicular cells, called thyroglobulin. One of the key steps in the production of thyroid hormones is the incorporation of iodine (i.e., iodination) into the hormone precursor thyroglobulin. This process necessitates the active uptake of iodide by the follicular cells of the thyroid (Figure 3). This utilization of iodide by the thyroid follicular cells is unique, as no other cells in the mammalian/vertebrate body require iodide. Ultimately thyroglobulin is broken down within the thyroid cells into the physiologically active thyroid hormones called thyroxine (i.e., T4) and triiodothyronine (i.e., T3) and then released into the blood stream. These thyroid hormones are ultimately responsible for setting the metabolic rate of the cells in the remainder of the body [2]. |
Figure 2. Microscopic image of the thyroid gland which reveals the spherical arrangement of the normal thyroid follicular cells surrounding the proteinaceous fluid, called the colloid that contains the precursor to the thyroid hormones T3 and T4 called thyroglobulin. The parafollicular cells that produce calcitonin are also evident in this image. |
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Figure 3. Illustration of a single thyroid follicular cell demonstrating the processes utilized to produce thyroid hormone. Thyroglobulin is synthesized in the endoplasmic reticulum and follows the secretory pathway to enter the colloid in the lumen of the thyroid follicle by exocytosis. Meanwhile, a sodium-iodide (Na/I) symporter pumps iodide (I-) actively into the cell. This iodide enters the follicular lumen from the cytoplasm by the transporter pendrin, in a passive manner. In the colloid, iodide (I-) is oxidized to iodine (I0) by an enzyme called thyroid peroxidase. Iodine (I0) is very reactive and iodinates the thyroglobulin at tyrosyl residues in its protein chain. In conjugation, adjacent tyrosyl residues are paired together. The entire complex re-enters the follicular cell by endocytosis. Proteolysis by various proteases liberates thyroxine and triiodothyronine molecules, which enters the blood. |
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