More About Congenital Hypothyroidism
In my last entry, I discussed a rare form of congenital hypothyroidism (CH) mostly to make a point about the importance of routine medical care in detecting potentially serious medical conditions. I did not really address much about general aspects of CH, something I will do now.
Why do we have newborn screening for congenital hypothyroidism?
The purpose of a screening test is to detect a specific medical condition in an asymptomatic patient. Screening tests are typically used if failure to make a timely diagnosis leads to a bad outcome. Obviously, there does need to be a test that can detect the condition without unreasonable frequencies of false positive and false negative tests (i.e., test results that falsely diagnosis the medical condition or fail to diagnose it). The part about “unreasonable frequencies” depends on many factors such as the cost of the screening test versus the cost of missing the diagnosis (e.g, chronic medical care costs for a missed diagnosis leading to a serious outcome).
CH is one of those medical conditions that most experts world-wide agree is “worth” screening for; we have an excellent and relatively inexpensive screening test, the number of false positive and false negative results is considered acceptably small (don’t ask me to quantify what “acceptably” means), and there is an inexpensive and effective treatment. Key to all of this is the fact that most cases of CH cannot be detected clinically until it is too late to prevent permanent central nervous system damage. Thus if the question is “what are the most common signs and symptoms in newborns with CH,” the correct answer is “nothing.” That’s why newborns screening for CH is so important.
“Classic” signs and symptoms of CH
Even though, most cases of CH are detected in timely fashion only because of newborn screening, there are some well-described signs and symptoms that are worth remembering. Signs include poor feeding, irritability, apnea spells, and constipation. Physical findings include large tongue, jaundice (yellowish skin from high levels of bilirubin), poor muscle tone, and edema (swelling of the hands and feet). The poor muscle tone is often exemplified by an umbilical hernia. Many other signs and symptoms have been described but remember, most often infants with CH provide no clues in the newborn period.
The timing of the newborn screening test
Immediately after birth, normal infants typically have a large “spike” in their TSH level (TSH is the pituitary hormone that controls the thyroid gland production of thyroid hormone). Thus, the TSH level used as a cut-off between normal and abnormal is much higher than what the upper limit of normal for the test would after the first week or so of life- after the TSH spike, the level does not return to normal until about day 4-5. Because, newborns are discharged from the hospital so quickly these days, it has become necessary to carry out 2 screening tests, one within 1-2 days of birth, and the other, 1-2 weeks later. This avoids the logistical (i.e., expensive) problem of too many false positive results. Of course, working out the details to be certain the follow-up screening test is performed, requires a high degree of organization in the medical care delivery system (it also requires a cooperative parent). This is one aspect of our shaky medical care system that seems to work pretty well- few cases of CH end up being missed in the U.S.
Most pediatric texts include a a disorder called “transient CH” in the differential diagnosis of CH. Apparently some newborns have abnormal TSH screens and on follow-up have normal thyroid function studies. In my experience, most of the cases are the result of newborn TSH spikes that were unusually high or testing that was too early but it is relatively easy to sort these cases out. If the blood level of thyroid hormone (T4) is normal and the TSH level is modestly elevated (i.e, <20 with a normal level <5), careful follow-up testing can usually sort things out and determine if the infant needs to be treated with thyroid hormone life-long or not at all .
- Congenital Hypothyroidism: Horses and Zebras
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