Thyroid

Thyroid Description

The thyroid is one of the larger endocrine glands in the body. It is located in the neck and produces hormones, principally thyroxine (T4) and triiodothyronine (T3), that regulate the rate of metabolism and affect the growth and rate of function of many other systems in the body.

Anatomy

The thyroid gland is located on the front part of the neck below the thyroid cartilage (Adam's apple). The gland produces thyroid hormones, which regulate body metabolism. Thyroid hormones are important in regulating body energy, the body's use of other hormones and vitamins, and the growth and maturation of body tissues.

Gross anatomy
The thyroid is situated on the front side of the neck at the level of C5 to T1 vertebral bodies, just below the laryngeal prominence (Adam's apple), near the thyroid cartilage over the trachea but covered by layers of skin and muscle. The thyroid is one of the larger endocrine glands - 10-20 grams in adults- and butterfly-shaped: the wings correspond to the lobes and the body to the isthmus of the thyroid. It may enlarge substantially during pregnancy and when affected by a variety of diseases.

Blood supply
The thyroid gland is supplied by two pairs of arteries: the superior and inferior thyroid arteries of each side. The superior thyroid artery is the first branch of the external carotid, and supplies mostly the upper half of the thyroid gland, while the inferior thyroid artery is the major branch of the thyrocervical trunk, which comes off of the subclavian artery. In 10% of people, there is an additional thyroid artery, the thyreoidea ima, that arises from the brachiocephalic trunk or the arch of the aorta. Lymph drainage follows the arterial supply.

There are three main veins that drain the thyroid: the superior, middle and inferior thyroid veins.

Histology of the thyroid
The thyroid is composed of spherical follicles that selectively absorb iodine (as iodide ions, I-) from the blood and for production of thyroid hormones. Twenty-five percent of all the body I- is in the thyroid gland. The follicles are made of a single layer of thyroid epithelial cells, which secrete T3 and T4. Inside the follicles is a colloid which is rich in a protein called thyroglobulin. The colloidal material serves as a reservoir of materials for thyroid hormone production and, to a lesser extent, a reservoir of the hormones themselves. Scattered among follicular cells and in spaces between the spherical follicles are another type of thyroid cell, parafollicular cells or C cells, which secrete calcitonin.

Thyroid Diagnosis

Some women may have thyroiditis that usually occurs within 3-6 months after birth. It also may occur after a miscarraige. The classic clinical picture is a woman who will first have symptoms of hyperthyroidism, followed by hypothyroidism, culminating in normal thyroid function.

The measurement of thyroid-stimulating hormone (TSH) levels is often used by doctors as a screening test. Elevated TSH levels can signify an inadequate hormone production, while suppressed levels can point at excessive unregulated production of hormone. If TSH is abnormal, decreased levels of thyroid hormones T4 and T3 may be present; these may be determined to confirm this. Autoantibodies may be detected in various disease states (anti-TG, anti-TPO, TSH receptor stimulating antibodies). Infrequently, TBG and transthyretin levels may be abnormal; these are not routinely tested.