Thyroid gland arises in embryo as a diverticulum of the floor of the pharynx which proliferates to form follicles and loses its connection with pharynx. In lower vertebrates it arises mid ventrally from between the second and fourth visceral clefts but in higher forms it develops between the first and second clefts. The thyroid has small follicles of cuboidal cells, the lumen of follicles is filled with an iodine rich colloid and the follicles are enclosed in a capsule of connective tissue.
This colloid secretion is a globular protein known as thyroglobulin which is also known as iodothyronins or thyroid hormones of which thyroxin is one.
Thyroxin regulates metabolism that is heat production and energy liberation in all the organ systems. The under secretion of this hormone lowers the rate of metabolism and causes the animal to become fatty, sluggish and feeble minded and even growth is retarded. The heart rate is also lowered and sexual development is retarded. This condition is known as cretinism. The deficiency of thyroxin may stimulate the formation of new gland tissue resulting in an enlargement of the thyroid called goiter. The goiter is actually caused by the deficiency of iodine in the diet. The over-secretion of this hormone increases the rare of oxidation and is responsible for the quick consumption of the food. Therefore fat is not stored. Over secretion also results in the bulging of eye balls and irregular heart rate, etc.
Parathyroid gland: these are small oval glands situated on the lobes of thyroid. These glands are either found embedded in the thyroid or located close to them. Parathyroid secretes two hormones which are responsible for regulating the amount of calcium and phosphorous in blood. The two hormones are parathormone and calcitonin. Over secretion of these hormones results in tumers on the gland and causes calcium to be withdrawn from bones, thus producing a high calcium level in the blood. In such conditions bone may become soft, porous and weak. Its deficiency lowers the calcium level in the blood due to which normal functioning of muscles bone growth and teeth formation are retarded. It has been seen experimentally that the removal of parathyroid result in the death of the individual.