ANATOMY AND PHYSIOLOGY OF THYROID GLAND
Thyroid Glandy y y y y y y y Is a butterfly- shaped organ
located in the lower neck anterior to the trachea. Consist of two
lateral lobe connected by an isthmus. The gland is about 5cm long
and 3cm wide and weights about 30g. One of the largest endocrine
gland. Appears more red than the surrounding tissues because it is
highly vascular. Thyroid gland contains numerous thyroid follicles.
Each follicles is filled with proteins to which thyroid hormones
are attached. Main function of the thyroid gland is to secrete
thyroid hormone which regulate the rate of metabolism in the
body.
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Growth and development cannot proceed normally without normal
rate of thyroid hormone secretions. Hormones produced by the
thyroid glands are the thyroxine(T4), Triidothyronine(T3) and the
thyrocalcitonin. These Hormones are synthesized and stored bound to
proteins in the cells of the thyroid gland until needed for release
into the blood stream. A lack of iodine in the diet results in
reduced thyroxine(T4) and triidothyronine(T3) synthesis. Thyroid
Hormones have a negative feedback effect on the hypothalamus and
pituitary gland so that increasing levels of thyroid hormone
inhibit the secretion of TSH- releasing hormone from the
hypothalamus and inhibit TSH secretion from the pituitary. When
thyroid hormone secretion is reduced below normal levels, the
secretion of TSH-releasing hormone from the hypothalamus and the
TSH-secretion from the anterior pituitary increases substantially.
Excess TSH causes the thyroid gland to enlarge, a condition called
Goiter. Iodine deficiency goiter develop if iodine in the diet is
too low. Less thyroxine(T4) and triidothyronine(T3) is synthesized
and secreted. Although serum thyroxine(T4) and triidothyronine(T3)
levels gradually increases or decreases together, the T3 levels
appears to be more accurate indication of hyperthyroidism, which
causes a greater rise in the T3 than RT4 level.
Calcitonine,thyrocalcitonine is another important hormone secreted
by the thyroid gland. It is secreted in response to high plasma
levels of calcium, and it reduces the plasma level of calcium by
increasing its deposition in bone
Source: Brunner &Suddarths, Medical-Surgical Nursing, Vol.1,
SmeltzerSzanne C. and Brenda C. Bare
Regulation Of Thyroid Hormone (T and T ) Secretion
1. TSH- releasing Hormones is released from neuron within the
hypothalamus into the blood. It passes through the
Hypothalamicpituitary portal system to the anterior pituitary. 2.
TSH-releasing Hormones causes cells of the anterior pituitary to
secrete TSH, which passes through the general circulation to the
thyroid gland. 3. The TSH causes increased released of thyroid
hormone (T and T ) into the general circulation. 4. T and T act on
target tissues to produce a response. 5. T and T also have
inhibitory effect on the secretion of TSH-releasing hormone from
the hypothalamus & TSH from the anterior pituitary.Source:
Essentials of Anatomy &Physiology,Sixthedition,Seeley, Stephen
and Tate
