This important gland of internal secretion is made up of two flattened lobes lying beneath the superficial muscles of the lower anterior part of the neck on either side of the trachea. The two portions of the gland are connected by a small bridge of thyroid tissue may sometimes be present along the length of the trachea. See figure below. During pregnancy and menstruation the thyroid may temporarily. increase in size. Complete removal and abnormal secretion of the gland causes grave systematic disturbances. The gland is also subject to tumor formation, which may be benign, i.e.
a cyst causing enlargement only, or malignant (solis tumor).
The function of the thyroid is to serve as a storehouse for iodine and to secrete into the blood stream thyroid hormone, which has a stimulating effect on growth and metabolism. The thyroid affects other ductless glands and the sympathetic nervous system. In a reverse manner, other endocrine glands in turn influence the thyroid; this is particularly true of the pituitary gland, which has a multiplicity of influences on the endocrine system.
Diseases of the thyroid may be classified as hypofunction, hyperfunction, tumors, goiter, cancer or inflammatory disease.
The hormone produced by the thyroid is now called thyroxine which exercises control over the rate at which food is converted into heat and energy in all the body’s cells. See figure below. Without sufficient thyroxine the individual feels constantly cold, drowsy, unable to do anything without considerable exertion. Respirations are slow, heart rate sluggish, appetite and sex functioning both below pars. Sometimes there is a weight gain despite a distinctly meager diet. The opposite of hypothyroidism is hyperthyroidism. An individual with this condition is likely to be nervous, jittery and overactive, with a pounding heart and labored respirations, able to gorge yet lose weight, as though all the body fires were burning out of control.
(figure available in print form)
Homeostasis of the Thyroid Gland
(figure available in print form)