The thyroid gland is part of and a major organ of the endocrine system. This gland is composed of two cone-like lobes connected together forming a butterfly shape ( shown in figure 1).The isthmus is a thin band of connective tissues which connects the two lobes of the thyroid gland is located between the pyramidal lobe and the trachea. (Cohen and Wood 2000) The thyroid gland is situated on the anterior part of the lower neck, below the larynx (voice box) and in front of the trachea (windpipe) (Britannica online 2011) where it could easily be palpated during a physical examination by a doctor. (Kumar and Clark 2005).
Due to it being the largest endocrine gland in the body, as it weighs approx 20g, it receives an extensive blood supply from thyroid arteries which gives it a deep red colour. (Cohen and Wood 2000) The two parathyroid glands are located between two layers of the capsule that covers the thyroid gland. It is an important feature as it monitors the circulating concentration of calcium ions in the blood. (Martini 2004)
The functional unit of the thyroid gland is the follicle, a spherical group of cells that form a colloid from a casing protein-rich storage material. (Nussey and Whitehead 2001) Thyroglobulin is a protein synthesised by follicle cells which contain amino acid tyrosine and are building blocks of thyroid hormones. Iodine is then needed for formation of the hormones by its absorption from the diet and then carried via bloodstream to the thyroid gland which is then secreted into the colloid and oxidised into iodine ions. The attachment of iodine ions to tyrosine molecules triggers the pairing process performed by the thyroid peroxidise. This leads to production of thyroxine hormone T4 (containing four iodine ions) and triiodothyronine T3 (containing three iodine ions). (Martini 2004)
Figure 1: The location of the thyroid gland. (Nussey and Whitehead 2001 )
The relationship between hypothalamus, thyroid gland and pituitary gland is significantly important as the hypothalamus is the main link between endocrine and nervous system. The nerve cells in the hypothalamus which is located in lower central part of the brain controls the pituitary gland. It does this by producing chemicals that stimulate or suppress hormone secretions from the pituitary gland. The hypothalamus has the highest level of endocrine control as it integrates the activities of the endocrine system. The integration involves the hypothalamus secreting regulatory hormones that control endocrine cells in the pituitary gland. (Nussey and Whitehead 2001 )
This gland is found beneath the hypothalamus which consists of two lobes (anterior and posterior) and is often called the master gland. The hormones released by the anterior lobe of the pituitary gland control the activity of the endocrine cells in thyroid gland. The regulation of thyroid hormone secretions in the thyroid gland is controlled by the negative feedback produced by both the hypothalamus and the anterior pituitary gland illustrated in figure 2. (Cohen and Wood 2000)
The concentration of TSH (thyroid-stimulating hormone) in circulating blood is a major factor that controls secretion rate of thyroid hormones. Its stimulation is influenced by TRH (thyrotropin releasing hormone) which is secreted by the hypothalamus into the anterior pituitary gland. This causes TSH to stimulate the production of thyroglobulin which leads to the release of thyroid hormones T4 and T3. However once the concentration of thyroid hormones exceeds the normal range, homeostasis is disturbed and signals are sent to hypothalamus to inhibit the secretion of both TRH and TSH. This decreases the hormones concentrations back to normal ranges and the secretion of TSH starts once again. Negative feedback is also generated when low levels of...