Screening for Congenital Hypothyroidism

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By Marie-Dominique Beaulieu
18               Screening for Congenital Hypothyroidism
                   Prepared by Marie-Dominique Beaulieu, MD, MSc, FCFP1

                              The Canadian Task Force on the Periodic Health
                        Examination concludes that there is good evidence to
                        recommend screening for congenital hypothyroidism in newborns
                        between 2 and 6 days of life. The maneuver consists of the
                        measurement of thyroid-stimulating hormone (TSH) levels in a
                        dried capillary blood sample, usually taken from the newborn’s
                        heel. Thyroxine (T4) level is measured if necessary. This
                        recommendation is unchanged from that made by the Task Force
                        in 1990,<1> except that, reflecting current thinking regarding test
                        characteristics, TSH is now considered the primary screening test
                        followed by T4, if necessary (previous recommendation
                        T4 followed by TSH).

                        Burden of Suffering
                               The incidence of congenital hypothyroidism ranges between
                        1 per 4,000 to 1 per 3,500 live births<2> and can be as high as 1 per
                        141 live births among infants with Down syndrome.<3> Before
Congenital              screening was available, the age at diagnosis ranged from 1 week to
                        5 years or more. The intelligence quotient (IQ) of 65% of patients with
occurs in 1 per
4,000 live births.
                        congenital hypothyroidism was below 85 (borderline intellectual
Intellectual deficits   functioning or lower), and in 19% it was below 15 (profound mental
develop in 65% of the   retardation).<4> It is estimated that about 30% of infants with delayed
cases without early     diagnosis would qualify for institutional care. Children whose mothers
treatment               ingested iodides, propylthiouracil or radioactive iodine or had
                        circulating antithyroid antibodies are at high risk for congenital

                               All industrialized countries now have well organized screening
                        programs for congenital hypothyroidism. Most screening programs in
                        Canada and in Europe use a two-step laboratory approach where TSH
                        is measured first and T4 in borderline cases. However, many programs
                        in the United States measure T4 first. The recall rate for T4 averages
                        1.1% with a positive predictive value of 2.4%.
                               Blood samples from the heel are collected on filter paper, ideally
                        3 to 6 days after birth, and tested for TSH concentration. TSH levels
                        after the first 24 hours of life can be as high as 20 mIU/ml, thus
                        increasing the number of false positive results. For this reason,

                            Associate Professor of Family Medicine, University of Montreal, Montreal, Quebec

newborns leaving the hospital in their first 24 hours of life are usually
not tested.
       The cut-off point for hypothyroidism is defined according to the
daily standard deviation of test results rather than by a predetermined
concentration. Since the laboratory method and the size of the blood
spot influence the TSH and T4 levels each program must define its
own standards and reference values. If the TSH level falls within
normal limits the T4 concentration is measured with another blood
spot on the filter paper. The recall rate with TSH testing averages
0.05%. At this rate, two infants are recalled for testing for every case
detected. The sensitivity of the combined maneuver is about 95%.
      Cases of primary, congenital hypothyroidism or about 5% of all
cases of congenital hypothyroidism cannot be identified with the TSH
screening approach.<5>

Effectiveness of Screening and Treatment
      The first evidence of the benefits of screening for congenital
hypothyroidism in Canada came with publication of the one-year
follow-up data from the Quebec Screening Program.<6>
       The mean IQ of affected infants less than 12 months of age was
greater than 100, and did not differ from that of control subjects. Both
the Quebec screening program and the New England Congenital
Hypothyroidism Collaborative have published five-year follow-up data
on children screened at birth.<7,8> The children who had been
treated for congenital hypothyroidism were comparable to matched
controls on all developmental scales. In the Quebec program, the IQ
of the children treated for congenital hypothyroidism was within
normal limits for children of their age, though the average IQ was
statistically lower than that of the control group (105 vs. 110).<7>
      Follow-up data up to 12 years after screening have recently been
published.<9> The IQ and developmental scores of the hypothyroid
children were still within normal limits. The degree of fetal
hypothyroidism at birth influenced the final outcome.
       The fact that most hypothyroid infants identified at birth by
screening have intellectual and psychomotor development in the
normal range, constitutes a dramatic improvement over the outcomes
in children previously diagnosed later in life.                             Screening and early
                                                                            dramatically decrease
Recommendations of Others                                                   the morbidity
                                                                            associated with
      Few Canadian organisations have issued recommendations on             congenital
screening for thyroid diseases. The U.S. Preventive Services Task Force     hypothyroidism
has recommended universal screening for congenital

                        Conclusions and Recommendations
                                The dramatic change in the natural history of congenital
                        hypothyroidism since the advent of screening warrants a grade A
                        Recommendation, even in the absence of a randomized controlled
                        trial. Thus, there is good evidence to recommend routine testing of all
                                Screening for congenital hypothyroidism is generally carried out
                        by hospitals and is therefore outside the control of most individual
                        physicians. Nonetheless, primary care physicians should make sure that
Newborns born at        all infants who are born at home or discharged from hospital within
home or discharged
                        24 hours after birth are tested for hypothyroidism within 7 days after
within the first
24 hours of life must
                        birth. It is better to obtain a specimen within 24 hours than no
be tested within the    specimen at all.
first 7 days of life

                              Since the last review of the Task Force on thyroid diseases was
                        published in 1990, a MEDLINE search between 1989 and 1993 was
                        conducted. The search was done using congenital hypothyroidism with
                        screening and prevention and control. Only original articles were
                        considered. The search yielded few new articles. Priority was given to
                        the highest levels of evidence according to the Task Force
                        methodology. This review was initiated in February 1993 updating a
                        report published in 1990.<1> Recommendations were finalized by the
                        Task Force in January 1994.

                        Selected References
                         1.   Canadian Task Force on the Periodic Health Examination:
                              Periodic health examination, 1990 update: 1. Early detection of
                              hyperthyroidism and hypothyroidism in adults and screening of
                              newborns for congenital hypothyroidism. Can Med Assoc J
                              1990; 142: 955-961
                         2.   Fisher DA, Dussault JH, Foley TP, et al : Screening for
                              congenital hypothyroidism: results of screening one million
                              North American infants. J Pedriatr 1979; 94: 700-705
                         3.   Postellon DC, Abdallah A: Congenital hypothyroidism:
                              diagnosis, treatment, and prognosis. Compr Ther 1986; 12:
                         4.   Klein RZ: Infantile hypothyroidism then and now: the results of
                              neonatal screening. Curr Probl Pediatr 1985; 15: 1-58
                         5.   Dussault JH, Morissette J, Letarte J, et al : Modification of a
                              screening program for neonatal hypothyroidism. J Pediatr 1978;
                              92: 274-277

 6.   Glorieux J, Dussault JH, Letarte J, et al : Preliminary results on
      the mental development of hypothyroid infants detected by the
      Quebec Screening Program. J Pediatr 1983; 102: 19-22
 7.   Glorieux J, Dussault JH, Morissette J, et al : Follow-up at ages
      5 and 7 years on mental development in children detected by
      the Quebec Screening Program. J Pediatr 1985; 107: 913-915
 8.   New England Congenital Hypothyroidism Collaborative: Effects
      of neonatal screening for hypothyroidism: prevention of mental
      retardation by treatment before clinical manifestations. Lancet
      1981; 2: 1095-1098
 9.   Glorieux J, Dussault J, Van Vliet G: Intellectual development at
      age 12 years of children with congenital hypothyroidism
      diagnosed by neonatal screening. J Pediatr 1992; 121: 581-584
10.   U.S. Preventive Services Task Force: Guide to Clinical
      Preventive Services: an Assessment of the Effectiveness of
      169 Interventions. Williams & Wilkins, Baltimore, Md, 1989:

         S   U    M   M      A   R Y        T    A   B      L   E     C    H   A      P   T   E   R    1 8

                 Screening for Congenital Hypothyroidism

  MANEUVER                       EFFECTIVENESS                  LEVEL OF EVIDENCE             RECOMMENDATION
  Measurement of                 Routine screening              Dramatic outcome              Good evidence to
  thyroid-stimulating            programs are effective         improvements in               ensure routine TSH
  hormone (TSH) level            in detecting preclinical       prospective cohort            testing among all*
  in dried capillary blood       hypothyroidism; early          studies<2-9> (II-2)           neonates (A)
  sample in first week of        treatment radically
  life; measurement of           modifies the mental
  thyroxine (T4) level if        development of
  necessary                      affected infants.

  *     Infants delivered at home or leaving the nursery within 48 hours may escape screening and
        must be reached otherwise.


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