From Screen to Scan: Navigating the Diagnostic Journey

The path from a newborn screening result to a clear diagnosis can feel like a series of hurdles. While the initial “heel prick” test is a powerful tool, it is only the first step. Understanding how doctors use follow-up tests and imaging can help you navigate the process of confirming exactly how your baby’s thyroid is—or isn’t—working.

Why Some Babies Need Repeat Screening

For most full-term babies, a single screening test a few days after birth is sufficient. However, if your baby was born prematurely (before 37 weeks) or with a low birth weight, their screening journey is different ^[1].

Premature infants often have an “immature” system that doesn’t always show signs of a thyroid problem right away. Their bodies may experience a delayed TSH rise, where the hormone levels that signal a problem don’t spike until weeks later ^[2][3]. Because of this, medical guidelines recommend that these babies receive repeat screenings to ensure a diagnosis isn’t missed ^[1][4]. The specific timing is determined by the local newborn-screening program and treating clinician based on risk factors, rather than a universal two- and four-week schedule.

The Limits of Standard Screening

It is important to know that most standard screenings are designed to find primary congenital hypothyroidism (a problem with the gland itself). They do this by looking for high levels of TSH (thyroid-stimulating hormone), the brain’s “shout” to the thyroid to work harder.

However, standard TSH-only screening can miss a rare condition called central congenital hypothyroidism ^[5]. In this version, the problem lies in the brain’s “command center” (the pituitary or hypothalamus). Because the brain isn’t signaling for more hormone, TSH levels appear normal on the screen even though the baby’s actual thyroid hormone levels are dangerously low ^[6][7]. If your baby shows symptoms despite a “passed” screen, doctors may need to order a more detailed blood test. Suspected central disease needs urgent endocrine assessment, including evaluation of cortisol/adrenal function before or alongside thyroid hormone treatment.

Symptoms to Watch For

Because screening is so effective, most babies are diagnosed before they ever look “sick.” However, if a screening is delayed or a result is borderline, you might notice signs of hypothyroidism.

• Urgent Red Flags: An infant who is extremely difficult to wake, refuses feeds, is dehydrated, has breathing difficulty, or is markedly lethargic needs urgent assessment. These are serious signs of illness. Additionally, pale stools, dark urine, or jaundice persisting beyond about 2 weeks in a term infant (or 3 weeks in preterm) require prompt medical evaluation.
• Subtle Signs: Other clues include a large anterior fontanelle (the “soft spot” on the top front of the head) ^[8], macroglossia (a large or thickened tongue) ^[9], an umbilical hernia (a small bulge near the belly button) ^[10], or persistent constipation ^[11].

Imaging: Ultrasound vs. Scintigraphy

Once a blood test confirms low thyroid levels, doctors use imaging to see what the gland looks like. There are two main tools, and they provide complementary information. Most importantly, imaging must never delay starting levothyroxine.

Ultrasound

An ultrasound uses sound waves to look at the structure of the neck. It is non-invasive and does not use radiation. However, it has limits:

• The “Empty Bed” Problem: An ultrasound may show no gland in the usual spot, but it cannot always tell if the gland is truly missing (agenesis) or if it is just hiding somewhere else ^[12].
• Mistaken Identity: Sometimes, muscles or other tissues in the neck can look like a small thyroid gland on an ultrasound, leading to an incorrect conclusion ^[13].

Scintigraphy (The “Function” Scan)

A scintigraphy scan (using technetium-99m or I-123) helps define the location and function of the thyroid tissue ^[14]. A tiny amount of a radioactive tracer is given to the baby, and a special camera sees where that marker “lights up.”

• Finding Ectopic Tissue: This scan is much better at finding a gland that “got lost” and ended up at the base of the tongue or elsewhere in the neck ^[12][15].
• Confirming Agenesis: While highly useful, absent uptake does not perfectly confirm agenesis, as it can be affected by recent iodine exposure, prior levothyroxine or TSH suppression, and technical limitations ^[13][16].

By combining these two tests when appropriate, your baby’s care team can determine if the thyroid is missing, misplaced, or simply too small, which helps them provide the most accurate long-term care plan.