RESEARCH PROJECT
CHAPTER ONE
INTRODUCTION
OVERVIEW OF THYROID HORMONE
Thyroid hormones are synthesized and secreted by epithelial cells of the thyroid gland (which is one of the largest endocrine gland) in the body (Broedel et al., 2003). Thyroid gland is a butterfly-shaped gland found anterior to the neck and inferior to the laryngeal prominence (Adams apple), which weighs at about 15grams to 20grams in adults (Noden et al.,1991; Walker and Liem, 1994).
It develops from an evagination of the floor of the neck and a thyroglossal duct marking the path of the thyroid, from the tongue to the neck , sometimes persists in the adult (Macchia, 2000).The two lobes of the human thyroid are connected by a bridge of tissue, thyroid isthmus .
The gland is well vascularized, receiving one of the highest rates of blood flow per gram of tissue of any other organ in the body.
Thyroid hormone is been synthesized and secreted by a spherical epithelial cells called Follicular cells (100 - 300micometers in diameter), which is filled with a pink staining proteinaceous material called Colloid. Paraphrasing, follicular cells is the functional unit of thyroid gland. (Gorbman and Bern, 1962)
1.1 THYROID GLAND HORMONE
Thyroid gland secretes two primary hormones:
(1) Tetraiodothyronine or Thyroxine (T4).
(2) Triiodothyronine (T3).
Tertraiodothyronine is a precursor, containing of about 80% to 90% of all thyroid secretion, with duration of action being four times more than triiodothyronine due to its high affinity to plasma protein.
Triiodothyronine contains of about 10% to 20% of thyroid secretion, is four times more active than T4 and has low affinity to plasma protein, thus lesser duration of action .Thyroid hormones are iodine containing derivatives of an amino acid called Tyrosine. (Frieden and Lipner 1971)
Thyroid gland also secretes calcitonin, an important hormone for calcium metabolism.
1.2 MECHANISM OF ACTION OF THYROID HORMONE
Thyroid hormone is a tyrosine derived hormone and executes its functions by acting on the genes-DNA (St German et al., 1988; Farwell et al., 1996)
(a) Hormone enters the interior of the cell and binds to the receptors in the nucleus, to form hormone – receptor complex.
(b) Hormone - receptor complex moves towards the DNA and binds to it.
(c) This increases transcription of RNA.
(d) The MRNA moves out of the nucleus and transported to the ribosome, activating protein synthesis.
(e) Activated protein in turns produces physiological responds.
1.3 THYROID HORMONE –ENZYMATIC RELATIONSHIP.
Iodotyrosine deiodinase contributes to the breakdown of thyroid hormones.It releases iodine, for renewed use, form iodinated tyrosines resulting from catabolism of iodothyronines.Iodotyrosine deiodinase employs a flavin mononucleotide cofactor and belongs to the oxidase/ flavin reductase superfamily.
Iodothyronine deiodinase catalyze release of iodine directly from the thyronine hormone (Bianco and kim, 2006).They are selenocysteine-dependent membrane protein with a catalytic domain resembling peroxiredoxins.(Van der Geyten et al., 1997) .
Three related isoforms , deiodinase type 1, 2and 3,contributes to activation and inactivation of the initially released hormone precursor T4 into T3 or rT3 in target cells(Galton, 2005; St German et al.,2005).
1.4 GENERALIZED EFFECT OF THE THYROID HORMONE
1. Increases basal metabolic rate by increasing oxygen consumption and body activities.
2. Food metabolism; Carbohydrate break down (gluconeogenesis), protein synthesis (anabolical effect) and fat mobilization.
3. Maintain normal erythrocyte synthesis.
4. Bone formation mediating osteogenesis, chondrocyte production and mineralization.
5. Maintenance of body weight .Hyperthyroidism leads to decrease in body weight
6. Enzymes formation by increasing vitamin concentration which is an important part of an enzyme.
7. Cardiovascular function; increases blood flow ,cardiac output, heart rate, heart strength, arterial blood pressure.
8. Increases rate of respiration by increasing oxygen utilization and carbon dioxide accumulation
9. Increases gastrointestinal tract motility, thereby increasing appetite and food intake. It also increases secretion.
10. Thyroid hormone helps in maintenance of muscular system due to its anabolical effects on protein. Hypothyroidism causes muscle sluggishness and relaxation while thyroid hormone causes vigor and tremor.
11. Thyroid hormone maintain sleep by acting on the reticular activating system. Hyperthyroidism causes nervousness while hypothyroidism causes somnolence.
12. Thyroid hormone has both general and specific effects on growth. In human the effects on growth is manifest mainly in growing children e.g bone and CNS growth and development
13. Central nervous system development e.g myelination, synaptogenesis, cell migration and differentiation.
Hypothyroidism can cause mental retardation while hyperthyroidism cause anxiety, complexes, extreme and paranoia, etc.
1.5 APPLIED PHYSIOLOGY OF THYYROID GLAND
HYPERTHYROIDISM: Excess synthesis and secretion of thyroid hormone. It is caused by the following: Graves’ disease (via synthesis of immunoglobulin-thyroid stimulating hormone), thyroid neoplasm, excess TSH secretion, thyroid gland adenoma etc.
SYMPTOM: Increase in basal metabolic rate, weight loss, negative nitrogen balance, sweating, goiter, exolphthaimus etc.
TREATMENT: Propylthiouracil, thyroidectomy and beta-adrenergic blocker etc. (Potter et al., 1986).
HYPOTHYROIDISM: Reduced thyroid hormone synthesis and secretion. It is caused by the following: Thyroiditis, iatrogenic origin, iodine deficiency etc.
SYMPTOM: Mental retardation, myxedema, drooping eye lid, cold sensitivity, anemia, cold tolerance, loss of libido etc.
TREATMENT: infusion of inorganic iodine (Potter et al 1986)
Used with permission.
Research done: Anyanwu Ifeanyi
Tel: +2348103101937
Facebook: Ifeanyi Hippolyte
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