Thyroid is an endocrine gland with internal excretion of its hormones. Thyroid hormones are thyroxine (T4) i triiodothyronine (T3). They affect all body tissues and they have an important role in regular function of all organs in the body. Disfunctional thyroid can excrete too much hormones T4 i T3, so the hyperthyreoidism occurs (that is a state in which thyroid metabolism is enhanced). When the thyroid excretes less T4 i T3 hormones than normally, the hypothyreoidism occurs. The worst complication of hypothyreoidism is a myxedema coma. Body tissues can also be resistant to thyroid hormones and that causes the hypothyreoidism. The hyperthyreoidism is generally manifested as Graves' disease, toxic nodule goiter or secondary hyperthyreoidism. Thyreotoxic crisis is the hardest complication of hyperthyreoidism which occurs suddenly. Goiter (nontoxic one) includes all types of thyroid's growth, regardless of the cause of the growth or therapy procedures which are different for different types of goiter. Goiter occurs when one or more factors decrease the thyroid's ability to produce and excrete enough hormones. Endemic goiter is generalized or localized growth of the thyroid in more of the 10% of the population. It is caused by one or more factors typical for a certain geographic area. The common reason is iodine deficiency but there are also other factors. Sporadic goiter is the thyroid growth in non-endemic area. Toxic multinodular goiter can be a consequence of a long-term simple goiter in endemic areas, while in non-endemic areas it is hardly detected. Toxic adenoma is similar to multinodular toxic goiter, but in this case there is a single node that causes hyperthyreoidism. Acute abscess, subacute and chronic Hashimoto's thyreoiditis are the most common thyroid abscesses. Thyroid tumors can be malignant or benign tumors.