Signs and symptoms of congenital hypothyroidism in the newborn are subtle [6] . An early diagnosis of the disease is only possible, in most cases, through newborn screening, dosing thyroid stimulating horm, one (TSH) levels in blood samples collected on filter paper, followed by measurement of thyroxine (total T₄) in serum sample, obtained from the heel, as shown in Figure 1 [7] -[10] .

TSH is the most sensitive screening test for establishing the diagnosis of primary hypothyroidism [11] . TSH dosage is also a useful tool to guide the replacement dose of thyroid hormone, except in patients with secondary hypothyroidism. In these patients free T₄ is the best method to be used. Adequate replacement of thyroid hormone results in normalization of serum TSH. Thyroxine is present in the serum in the free form (free T₄) and connected to TBG (total T₄) [12] .

Assays for total T₄ measure it in both states and are useful for establishing the diagnosis of hypothyroidism and assess response to treatment. The serum total T₄ and free T₄ are low in patients with primary hypothyroidism.

In the case of juvenile hypothyroidism is important to the check the presence of serum antithyroid antibodies that are useful only to establish the diagnosis of chronic lymphocytic thyroiditis or Hashimoto’s thyroiditis [13] , the most common cause of hypothyroidism and goiter in children [2] [14] . Titles antithyroglobulin thyroid anti-

bodies (anti-Tg) and antiperoxidase (TPO) are present in 85% to 90% of children with Hashimoto’s thyroiditis.

Children diagnosed with acquired hypothyroidism, having received appropriate treatment at least five years before the onset of puberty, often grow to reach adult final height consistent with their genetic potential [13] [15] .

In Brazil, the Neonatal Screening Program uses the TSH cut-off value of 10 mIU/L (immunofluorimetric method). When the TSH values are above 20 mIU/L, the newborn must undergo a medical check, where it will be examined and prompted a collection of blood for serum TSH and total T₄ and/or free. When TSH levels are between 10 mIU/L and 20 mIU/L, the newborn is called for a second test on filter paper [10] .

Most patients, in this second test, show normal results and do not require further monitoring. On those with TSH levels below 10 mUI/L, there is no need for a follow-up. TSH levels in unaffected children may be higher during the first 24 hours, due to the stress of labor, but usually normalize within 72 hours [9] [10] .

Some programs suggest reducing the TSH cutoff for 5 mIU/L to 6 mIU/L, as this would enable the diagnosis of some cases of Congenital Hypothyroidism undetected with 10 mUI/L limit, but some authors question the benefit of reducing this TSH limit, since there is no evidence of cognitive impairment in patients with TSH levels between 5 mIU/L and 10 mIU/L, and moreover, this strategy may increase the number of false negatives, and with it the number of recalls and treated children diagnosis and all financial and emotional cost associated [14] .

Despite the collection of newborn screening being performed by the 5th day of birth, the doctor only gets the results in the first query that occurs in the first month. It is important that the health unit, through the nurse, goes to the home of the newborn within the first seven days of life, to guide the family about all the symptoms related to chronic diseases screened in the test, and to remind of the importance of the introduction of vitamin D, which should remain until the second year of life or according to the physician’s discretion, assisting in the prevention of various diseases [13] .

In the flow chart presented by Figure 2, there is a didactic and practical for clinical use, facilitating the cutting points and conducts in specific cases.

The main purpose of treating congenital hypothyroidism is to keep the patient in an euthyroid clinical condition, providing thus normal growth, adequate psychomotor and pubertal development.

Treatment consists of daily oral administration of thyroid hormone in the morning. The standard substance used is sodium levothyroxine (sodium salt of synthetic isomer of thyroxine-T₄), orally and fasting, for better absorption and efficacy of the medication. It has a half-life of seven days, favoring the administration once daily. Only seldom, when a clinical condition restrains oral use, it needs to be done by parenteral route (in this case, use is made with 75% to 80% of the recommended dose orally). Treatment should be started as early as possible, preferably before 13 days of life, to prevent clinical consequences of lack of thyroxine [16] .

The dose varies depending on the age of the newborn and their body weight. Young children require higher doses than older children and adults. It should begin with doses of 10 - 15 mg/kg/day for the newborn to term, and the dose should be recalculated as the child’s weight gain and T₄ and TSH levels are observed in laboratory

tests [17] . The first objective, taking high dose thyroxine levels, is to reach the upper normal range within 1 - 2 weeks.

Serum TSH concentration may not return promptly to normal levels, even when using high doses of thyroxine hormone due to the feedback system; however, after 2 years of age, the levels tend to normalize, and TSH becomes an adequate treatment goal. Ideally, the levels of free T₄ during the first 10 year of life levels remain stable, preferably in the upper half of normality, as shown in Table 1.

It is important to confirm, always at each visit, adequate adherence to treatment. Guiding the family about the importance of regular treatment is essential. Warrant is needed if the children remain in a hypothyroid condition, because growth and development potentially will be adversely affected. It is not advised to fraction the drug tablet [18] . Currently there are the following commercial presentations of levothyroxine in the Brazilian market: 25 mcg, 38, 50, 75, 88, 100 112, 125, 150 and 200 mcg/tablet.

An overdose of levothyroxine sodium is undesirable, and should be avoided not only because of the symptoms of thyrotoxicosis (irritability, insomnia, intense crying, tremors, sweating, loose stools and tachycardia) but also because of the possibility of promoting advancement of bone age, and because it increases the risk of premature closure of sutures the cranial (craniosynostosis), when higher doses are maintained for prolonged periods [19] .

It is essential to individualize the dosage of levothyroxine for each patient, through a rigorous follow-up control, involving clinical monitoring that seeks to identify signs and symptoms suggestive of inadequate dosages.

In addition to the clinical evaluation, it is necessary to keep a hormonal control obtained through laboratory tests that provide a high level of confidence for the prescription dosage of the drug.

Other indicators of the effectiveness and treatment efficiency can be obtained with the control of physical growth and maturation, as well as the child’s developmental level (Güell, 1998) [20] .