Origin. The thyroid gland originates from the endoderm of the embryo.
Thyroid gland Location. The thyroid gland surrounds the front of the larynx and upper part of the trachea in the neck .
The thyroid is the largest endocrine gland. It is a bilobed, highly vascular organ. The two lobes are connected by a narrow band, the isthmus. It is composed of rounded follicles held together by loose connective tissue, the stroma, and enclosed by a thin fibroelastic capsule. The stroma contains blood capillaries and small groups of parafollicular or ‘C cells (clear cells). A follicle has a wall of cuboidal epithelium resting on a basement membrane, and is filled with a gelatinous colloid secreted by the epithelium.
Thyroid gland Hormones.
The thyroid gland secretes three hormones: thyroxine triiodothyronine (T3) and calcitonin.
Activation. Thyroid is stimulated to secrete its hormones by thyrotrophin, a hormone from the anterior pituitary (mentioned ahead).
(i) and (ii) Thyroxine and Triiodothyronine.
Both these hormones are iodinated forms of the amino acid tyrosine. They are stored in the colloid that fills the follicles, and are released to the blood when needed. The stored hormones are enough to supply a person for upto 10 months. Thyroid is the only endcrine gland whose product is stored in such a large quantity. The storage occurs in an unusual place, the extracellular colloid.
The hormones of the colloid have 4 functions –
(a) They control the general metabolism by regulating the rate of oxidation and production of energy. They maintain the basal metabolic rate (BMR) of the body.
(b) They promote growth of body tissues and development of mental faculties.
(c) They stimulate tissue differentiation. Therefore, they affect metamorphosis of a tadpole into an adult frog.
(d) Thyroxine also controls the working of kidneys. Its deficiency results in decreased urine output and vice versa.
(iii) Calcitonin (CT) . It is secreted by the C cells. It regulates the concentration of calcium and phosphorus in the blood . It is under the feedback control of plasma calcium concentration, and is secreted when concentration of calcium rises in the blood. It then lowers the concentration of calcium and phosphorus in the plasma by decreasing their release from the bones.
Thyroid gland Disorders symptoms and treatment
Improper secretion of the above thyroid hormones causes the following disorders :
(i) Grave’s Disease (Exophthalmic Goitre).
It is caused by hypersecretion (over secretion) of thyroid hormones due to enlargement of thyroid. Excess of thyroid hormones increases metabolic rate and accelerates oxidation. This results in quick consumption of food, leaving nothing for storage and causing emaciation (excessive leanness). Increased oxidation leads to quick heart beat, rise in blood pressure, higher body temperature, nervousness, irritability, tremor (trembling) and bulging eyeballs (exophthalmos). The disorder can be rectified by removal of a part of the gland.
It is caused by hyposecretion (under secretion) of thyroid hormones in infants. A cretin has markedly reduced metabolic rate and oxidation. This leads to slow heart beat; lower blood pressure; decrease in body temperature ; stunted growth ; mental retardation ; awkward body with pot belly, protruding tongue and pigeon chest*; and retarded sexual development. The disorder can be treated by an early administration of thyroid hormones (hormonotherapy).
It is caused by deficiency of thyroid hormones in adults. It is more common in women than in men. It is characterized by puffy appearance due to accumulation of fat in the sub-cutaneous tissue because of low metabolic rate and retarded oxidation. The patient lacks alertness, intelligence and initiative. He also suffers from slow heart beat, lower blood pressure, decreased body temperature and retarded sexual development like a cretin. Myxoedema can also be treated by administration of thyroid hormones.
(iv) Iodine Deficiency Goitre.
It is caused by deficiency of iodine in the diet. Iodine is needed for the synthesis of thyroid hormones. The latter suffer shortfall with poor iodine supply. The disease is common in hilly areas. It causes enlargement of thyroid gland. Addition of iodides to the table salt prevents the disorder. It may lead to cretinism or myxoedema. Overactivity of thyroid is known as hyperthyroidism, and underactivity as hypothyroidism.