Hypothyroidism is a common endocrine disorder caused by reduced thyroid function, or lack of effect of thyroid hormone in target cells. An estimated 1-2% of women and 0.2% of men have this common thyroid function disorder. The prevalence of subclinical hypothyroidism is as high as 5-20% in older women. Women are six to fifteen times more likely to develop hypothyroidism than men men. Occurence is less frequent in children and the elderly. Hypothyroidism can be congenital or acquired during life. Due to endocrine dysfunction it can be primary, secondary or tertiary, and according to the clinical picture hypothyroidism presents as prominent, mild or subclinical. Clinical signs and symptoms of hypothyroidism are caused by the decreased effect of thyroid hormones in peripheral tissues. General symptoms and signs of include chills, decreased sweating, weight gain, constipation, weakness, exhaustion, drowsiness, slowed movements and speech. The most common cause of hypothyroidism is chronic autoimmune thyroiditis or Hashimoto's thyroiditis. It is an autoimmune thyroid disease characterized by lymphocytic infiltration and destruction of thyroid tissue. Measurement of serum thyrotropin (TSH) levels is the first test for the detection of hypothyroidism according to Croatian Thyroid Society guidelines, as well as other national guidelines. TSH is the most sensitive test for detecting mild (subclinical) thyroid dysfunction, and an excellent screening test for hypothyroidism. In Croatia, as in most countries, newborn screening for hypothyroidism is conducted by measuring TSH from umbilical cord blood. This has led to the early identification of many cases and thus the prevention of developmental delay. Hypothyroidism is a permanent condition, regardless of cause, and therefore requires lifelong substitution therapy. Hypothyroidism is treated with Lthyroxine in amounts which meet metabolic needs. Doses range from 50-200 mg daily and are adjusted according to symptoms and normalization of thyroxine and TSH levels.