The butterfly shaped thyroid gland is a bilobed gland located below the laryns on each side of upper trachea. It is the largest endocrine gland in the body. Its two lateral lobes are connected by a median tissues mass called isthmus. Is slope is made up of many lobules. The lobules consists of follicles called acini. Each acinus is lined with glandular, cuboidal or squamous epithelial. cells. The lumen of acinus is filled with colloid, a thick glycoprotein mixture consiting of thyroglobulin molecules.
Hormones of the thyroid gland are often called the major metabolic hormones. The follicular cells of thyroid gland secrete two hormones namely tri-iodothyronine `(T_3)` and thyroxine or tetra-iodothyronine `(T_4)`. THe parafollicular cells or .C. cells of thyroid gland secrete a hormone called thyrocalcitonin. Iodine is essential for the normal synthesis of thyroid hormones . Thyroid releasing hormones form the hypothalamus stimulates the adenohypophysis to secrete .TSH., which inturn stimulates the thyroid gland to secrete the thyroid hormones. Thyroid hormones show a negative feedback effect on the hypothalamus and pituitary.
