Congenital hypothyroidism is one of the preventable causes of mental retardation if diagnosed within first 3 months of life. Major complications can be prevented if diagnosed early and started on replacement therapy. Early diagnosis is difficult because the clinical signs are not apparent at birth and appear gradually. Thus it is very important to screen newborns for congenital hypothyroidism and it is a routine newborn screening test in most of the developed countries.

SCREENING METHODS :

Specimens are obtained by heel stick filter-paper blood spot. The results are more reliable if TSH test is done on day 5 of life and T4 test between day 2 and 5 of life. There is a physiologic increase in T4 levels at 2-5 days of life, which is not seen, in hypothyroid infants.

T4 and sequential TSH measurement :

Most North American programs use this method. First, a filter paper blood test for T4 is done. All with low T4 values a TSH measurement is done.

Advantages :
Detects neonates with :

• Primary hypothyroidism which has low T4 and high TSH
• Thyroxine binding globulin deficiency with low T4 and a low TSH. TBG concentration measurement should be done in such cases.
• Hypothalamopituitary hypothyroidism, which has low T4 and normal TSH.
• Can detect secondary and tertiary hypothyroidism, which cannot be detected by TSH screening alone.
• Detects most cases of compensated hypothyroidism.
• Cheaper
  
  

Limitations :

• Fails to detect infants with normal T4 and elevated TSH.
• High false positive results because T4 levels are affected by changes in the concentration of TBG. False positive results are seen in preterm and stressed infants.

TSH and sequential T4 measurement :

Most laboratories in Europe use this method. TSH is measured in all newborns and it is followed by T4 in cases with high TSH values. This method is used when detection of primary hypothyroidism is the major goal.

Advantages :
Less variable results resulting in fewer false positive cases.

Limitations :
This method will miss cases with :

• Thyroxine-binding globulin deficiency
• Hypothalamo-pituitary hypothyroidism

The diagnosis of congenital hypothyroidism must be confirmed by measurement of serum T4 and TSH concentrations at 2-6 weeks of life.