In anatomy, the thyroid (IPA 'θairoid) is an endocrine gland. It is situated on the front side of the neck, just below the Adam's apple, near the thyroid cartilage over the trachea but covered by layers of skin and muscle. The thyroid is quite large for an endocrine gland - 15-30 grams - and butterfly-shaped: the wings correspond to the lobes and the body to the isthmus of the thyroid.
The primary function of the thyroid is production of hormones:
Thyroxine is critical to the regulation of metabolism and growth, throughout the animal kingdom. Among amphibians, for example, administering a thyroid-blocking agent such as propylthiouracil can prevent tadpoles from metamorphosing into frogs; conversely, administering thyroxine will trigger metamorphosis.
In humans, children born with thyroid hormone deficiency will not grow well, and brain development can be severely impaired, in the condition referred to as cretinism. Newborn children in many developed counties are now routinely tested for thyroid hormone deficiency; this is done by analysis of a small drop of blood from the child (usually, the blood also is tested for phenylketonuria and several other metabolic diseases of genetic etiology). Children with thyroid hormone deficiency are easily treated by supplementation with synthetic thyroxine, which enables them to grow and develop normally.
Because of the thyroid's selective uptake and extreme concentration of what is actually a quite rare element, it is extremely sensitive to the effects of various radioactive isotopes of iodine produced by nuclear fission. In the event of large accidental releases of such material into the environment, the uptake of radioactive iodine by the thyroid can, in theory, be blocked by saturating the uptake mechanism with a large surplus of non-radioactive iodine, taken in the form of iodide tablets. While biological researchers making compounds labelled with iodine isotopes do this, in the wider world such preventive measures are usually not stockpiled before an accident, nor are they distributed adequately afterward - one consequence of the Chernobyl disaster was an increase in thyroid cancers in the years following the accident. 
The most common diseases of the thyroid: