Scientists that study the effect of age on thyroid-stimulating hormone (TSH), Free T4 (FT4) and Free T3 (FT3) thyroid hormones often exclude the age group from birth up to age 16, 18, or 19.
They usually exclude children and teenagers because they know they are different. Describing their TSH and thyroid hormones is its own complex topic. The effect of age (and at times, biological sex) on the development of the young hypothalamus-pituitary-thyroid (HPT) axis is quite remarkable.
However, when people exclude youth from discussions of “population-wide” studies of thyroid hormones and “age-specific” differences, it makes it seem like the only important age-specific shifts in the HPT axis occur between younger adults, middle-aged adults, and seniors.
Even if you are not reading as a parent, pediatrician, or a teenager, there are many good reasons to learn from this area of thyroid science.
Two practical reasons:
- To prevent the misdiagnosis and delayed diagnosis of children and youth with thyroid disorders, pituitary TSH disorders and metabolism disorders, and
- To raise your voice in advocacy for age-specific reference intervals for TSH screening for primary thyroid disease. Many laboratories still refuse to provide age-specific ranges.
Children and youth are part of humanity, not special or divergent cases. One should not make sweeping generalizations about “the” HPT axis and “the” TSH–FT4 relationship without observing how young people’s TSH, FT4 and FT3 levels and relationships differ from adults.
This age group’s transformations illustrate key principles about the flexibility of the HPT axis:
- None of the childhood reference ranges and medians align well with those of adults.
- TSH and FT3 fluctuate the most across childhood, while FT4 declines slowly after 6 years of age.
- FT3 levels are much higher in youth than in adults, but without causing hyperthyroidism. The median and lower limit of FT3 intervals do not fall as low as adults, proving that they require more FT3.
- During puberty from age 12 to 19, male and female levels for FT3 diverge significantly as females lose FT3 while maintaining FT4 and TSH.
In this article, I prioritize material from researchers who studied all three hormones — TSH, FT4 and FT3. The free fraction of the most powerful thyroid hormone, T3, does not always align with Total T3 (TT3) levels. Many transporters have a higher affinity for Free T3 than for Free T4. Cells that do not express any enzymes that convert T4 to T3 will rely 100% on Free T3 for their signaling needs. Therefore, Free T3 is necessary for a complete understanding of HPT axis function and peripheral tissue thyroid hormone status — especially in children and youth, whose FT3 levels differ from adults.
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Preface: TSH screening vs. full diagnosis
Thyroid hormone dysregulation has profound effects on children’s mental and physical health and development. Therefore, it’s vital to find as many cases as possible, and as soon as possible.
A young person’s thyroid diagnostic process may be derailed by misusing adult reference intervals for TSH. Hypothyroidism or hyperthyroidism may be falsely ruled out on the basis of a “high-normal” or “low-normal” TSH result for an adult, which would be “abnormal” for a child.
Pediatric FT4 and FT3 ranges are necessary as well, since a child’s final diagnosis cannot rest on a TSH level alone. Leading thyroid researchers at the US National Institutes for Health (NIH) state the following:
“TSH alone should not serve as the gold standard for the measurement of thyroid status.
More accurate assessment is afforded through additional mass spectrometric measurement of thyroid hormones in combination with physician assessment of the patient’s clinical condition.”(Sheikh et al, 2018)
Therefore, a final and complete diagnosis relies on analyzing TSH, FT4 and FT3 levels and relationships.
Until a future date when “mass spectrometric measurement” (LCMS/MS) assays for FT3 and FT4 become widely available, clinicians will have to rely on the immunoassays commonly available at laboratories, while acknowledging the fact that low-normal FT4 and FT3 levels may actually be lower if measured by LCMS/MS.
Caution: While looking at these graphs and tables below, keep in mind that age-specific reference intervals for FT3 and FT4 will be unique to each laboratory assay platform. The measurements of thyroid hormone immunoassays by Cobas Roche, Advia Centaur, Abbott, and others differ considerably from each other because they have not yet been calibrated to the international gold standard mass spectrometry assay (Thienpont et al, 2010).
Fortunately, calibration does not affect the precision or reliability of the assay when one is relying on a single laboratory and their chosen assay platform. It only makes it difficult to translate results from one assay platform to another.
Neonatal TSH, FT4 and FT3 levels
First, let’s look at the early phases of life.
There is a neonatal surge in TSH and FT4, which is why TSH is usually not measured in the few hours following birth:
“Immediately after birth, there is a surge in TSH secretion which peaks at around 30 minutes postnatally, with levels as high as 60 to 80 mU/L.
This is followed by an increase in T4 which in turn peaks during the first day of life, when free T4 levels which can be twice as high as in older children or adults.
TSH and free T4 levels fall during the first month of life, but can still be above levels seen in adults.”(Walsh, 2022)
In 2018, Jayasuriya and colleagues measured neonatal TSH and free thyroid hormone levels in 569 babies who were not judged to be hypothyroid. They used a Beckman Coulter assay.
Jayasuriya and colleagues explain how different the TSH levels are from adults’ levels:
“compared to adults, the upper limits of TSH concentrations are approximately five- to eight-fold higher in the 48 h of life.”
They also explain why it is important to have daily and hourly intervals in this phase of life:
“Use of hour-based RIs [reference intervals ]in newborns allows for more accurate identification of neonates who are at risk of hypothyroidism. […]
The narrow RIs beyond 48 h of life support the current practice of a newborn screening test at days 3–4 of life.“(Jayasuriya et al, 2018)
A recent study by Naafs and colleagues (2020) confirmed that this powerful TSH stimulation produced higher FT4 levels in the first 15 days of life.
Interestingly, TSH is maintained from day 5-15 while FT4 declines and narrows, showing the unusual behavior of the human HPT axis at this age.
The inverse relationship would have revealed the high TSH together with a lower FT4. As shown in Kapelari’s and Naafs’ graphs above, however, the slopes of the median (50th percentile) reveal that both TSH and FT4 are increased compared to adult reference ranges. This is a positive relationship, not a negative one.
The positive relationship may be explained by the fact that children’s TSH hormone molecules have “high bioactivity” compared to adults, so they provide extra stimulation to a healthy young thyroid (Walsh, 2022).
Children have a higher demand for thyroid hormone per kg of body weight
Overall, children appear to have a higher demand for thyroid hormone than adults, as proven by clinical experience with hormone replacement:
“Children who develop hypothyroidism require higher doses of levothyroxine per kilogram (kg) of body weight than do adults, approximating
10 to 15 µg/kg/day in neonates,
4 to 6 µg/kg/day from age 1 to 5 years, and
2 to 4 µg/kg/day in older children and adolescents, whereas adults typically require 1.6 to 2.0 µg/kg/day.
This indicates the high level of metabolic and secretory activity of the thyroid during childhood.”(Walsh, 2022)
This increased demand only partially explains their higher FT4 and FT3 levels.
Where do these higher levels of hormone go? Babies also have a higher urinary clearance rate for TSH and Free T4 that strongly correlates with their circulating levels, on average (Cao et al, 2010).
Kapelari’s 2008 data from birth to age 18
One of the earliest and largest studies of children’s free thyroid hormones and TSH was performed by Kapelari and colleagues in 2008. Their data set included 2,194 serum samples from 1,495 patients admitted to the pediatric clinic of a hospital. They used an Advia Centaur assay.
The strength of this study was not only its size, but also
- the inclusion of TSH, FT3 and FT4 results beginning a few days after birth,
- the comparison of male and female reference intervals
- the visualization of various percentiles within the reference range so that one can see the data distribution.
Exclusion criteria were as follows:
- No diagnostic codes for health conditions known to affect thyroid function, such as thyroid disorders, eating disorders, pituitary disease, or genetic disorders.
- No use of drugs known to affect thyroid function.
- No children with a height or weight deviation beyond normal.
- Excluded 32 patients’ TSH data that revealed initially elevated TSH, but normal FT3 and FT4, while their clinical status was euthyroid. (These patients had a follow-up normal TSH test which was included in the data.)
Therefore, not every blood sample was screened for thyroid antibodies, and only certain illnesses and drugs were excluded.
An interesting paradox is that in the first 12 months of life, the expected negative inverse relationship between TSH and FT4 seems to be greatly diminished or blunted.
In contrast to the differences caused by age, the differences between male and female sex were insignificant for TSH and FT4.
The researchers provided two separate graphs for FT3 because they differed by sex as well as age:
- The FT3 levels in the first month of life are significantly higher for males.
- After age 10, the females lose FT3 while the males maintain or gain this hormone. (This is reflected in later studies, shown below)
The call for age- and sex-specific reference intervals
Kapelari and team made a persuasive argument:
“In children and adolescents, reliable age- and sex-specific reference intervals are an indispensable prerequisite to interpret individual thyroid function correctly.” […]
“Apart from the age group 15 – 18 years, differences were observed to the published reference intervals for adult populations, especially for TSH. …
“Applying this [adult TSH] upper range to pediatric patients might lead to a considerable high number of false diagnoses of hypothyroidism and result in unnecessary lifelong replacement therapy.”(Kapelari et al, 2008)
Kapelari forgot to mention that false negative diagnoses of hypo- and hyperthyroidism would also occur, as outlined above in the discussion of Iwaku’s 2013 research.
False negative diagnoses are arguably more harmful than false positives, because an “unnecessary” therapy can still be adjusted to promote health, but lack of a “necessary” therapy raises health risk and reduces quality of life until it is detected and treated.
Any system that aims to protect children’s health ought detect even the early and mild cases in children who are outside their TSH 95% population interval in case they progress quickly to dysthyroidism.
Adult reference ranges on the same Advia assay platform are approximately as follows:
- TSH: 0.3–3.5 mIU/l, as measured by the same research group in an earlier study (Moncayo et al, 2007)
- FT4: 10–20 pmol/L, as measured by a different group of researchers (Barth et al, 2016)
- FT3: 3.5–6.4 pmol/L as measured by a different group of researchers (Mirjanić-Azarić et al, 2022)
Iwaku’s 2013 study
The benefit of this study was its quantitative comparison of the ranges of children/youth with those of adults on the same assay platform.
Its two main limitations were:
- It did not examine sex differences, and
- It only examined a narrow band of ages from age 4 to 15.
The scientists recruited 342 children (111 males, 231 females) examined in a Japanese hospital over approximately 9.5 years, and it measured hormones using an ECLusys Roche assay.
Exclusion criteria were:
- No goiter; no thyroid nodules, including micronodules, or normal thyroid patterns, no atrophy or abnormal echogenicity
- Negative for anti-thyroid antibodies (TgAb, TPOAb).
- No congenital anomalies of the thyroid gland
- No current/past history of disorders of the thyroid gland.
Therefore, the study still included illnesses and drugs known to distort the HPT axis.
The researchers usefully provided vertical scatter plots for TSH levels in each age group. The adult reference interval is represented by the shaded area.
As one can see in the graph above, misusing adult TSH ranges to screen children for thyroid disease would result in a significant percentage of false negatives and false positives:
- Children aged 11–14 with a TSH between 3.0–4.5 mIU/L will have a false negative “normal” TSH screening result. Their FT4 should be verified in case of potential hypothyroidism.
- Children aged 4–12 with TSH below 0.53–0.67, which is low for their age, may still be within the “normal” TSH range for adults, causing a false negative “normal” screening result. Their FT4 and FT3 should be verified in case of potential hyperthyroidism.
- Children younger than 10 years old may have a TSH mildly higher than adults, but this may be “normal” for their age, causing a false positive TSH screening result for potential hypothyroidism.
The next graphic is derived from Iwaku’s table of reference ranges, converted to pmol/L (International System of Units, “SI units”).
The graphic visualizes the shifting “trio” of TSH, FT4 and FT3 reference intervals within each age group.
Patterns to notice in the graph above:
- The graphic reveals an overall declining pattern in all three hormones, with some exceptions.
- The “adult” ranges are wider because they attempt to accommodate a wide diversity of adult age groups from 20 to 80+ years old.
- Until age 10, the higher-normal levels of FT3 and FT4 do not reduce the lower-normal end of the TSH interval, revealing that the usual pituitary response to high-normal thyroid hormones is dampened.
- In contrast, from age 11–15, a TSH interval that does not rise as high correlates with a fall in the lower end of FT3 and FT4 intervals, revealing that the usual pituitary response to low-normal thyroid hormones is dampened.
Below is the original table from which the graph above with conventional units (pg/mL, ng/dL), showing the conversion factors for SI units (pmol/L) that I used to create the graph above.
Implications for diagnosis of children and youth:
- A child with a FT4 (or both FT3 and FT4) in the lower end of the adult reference ranges is hypothyroid, regardless of where the TSH falls.
- A child with a FT3 mildly above the adult reference interval is not likely to be hyperthyroid or in a state of “T3-toxicosis.”
The researchers called for age-specific reference intervals:
“It is, therefore, of importance to carry out evaluation of the thyroid function in children using reference values appropriate for the chronological ages, because
• misdiagnosis of hypothyroidism or inappropriate secretion of TSH (SITSH), especially in children aged 10 years or younger, and
• oversight of mild subclinical hypothyroidism which could occur at an increased incidence among children aged 11 years or older
could occur if the diagnosis is made using reference values for adults.”(Iwaku et al, 2013)
However, at any age, one should never presume that a TSH level just above the reference interval makes hypothyroidism “mild” or “subclinical.”
“A number of factors contribute to the diminished utility of TSH as a primary diagnostic indicator for thyroid disorders. ….
The direct measurement of thyroid hormones via LCMS/MS more closely correlates with the patient’s condition.”(Sheikh et al, 2018)
TSH is an imperfect screening test, not a primary diagnostic test. This means that not the TSH, but the FT3 and FT4 levels (when measured by an accurate assay, by age-appropriate reference intervals), determine how mild or severe the hypothyroidism is for the heart, liver, kidney, GI tract, cerebral cortex, and so on.
Currently, children with primary hypothyroidism benefit from the diagnostic category of “subclinical primary hypothyroidism” because it often triggers a TPO antibody test and a full FT3 and FT4 assessment.
Unfortunately, no one has yet developed a diagnostic category for “subclinical central hypothyroidism.” Such a category would recognize that some people have a very low-normal FT4 alongside an inappropriately normal TSH for their age and sex. An early diagnosis matters because a pituitary TSH secretion failure may progress over time.
Gunapalasingham’s 2019 study
A recent study by Gunapalasingham and colleagues provided a very large sample.
Although it excluded ages 0 to 5, it provided a lot more detail on age and sex effects between age 6 to age 19, using scatterplot graphs that covered each year of life.
They measured 2411 healthy school children in Denmark using two assays, 1) Cobas Roche and 2) Siemens, and they found that assays’ results were concordant.
Click to view assay methods and population exclusions
In the graphs below, their separate male and female graphs have been overlaid to provide a visually concise comparison.
In contrast with Kapelari’s and Iwaku’s graphs above, Gunapalasingham’s TSH graph is very flat, partly because the age begins at 6 years old and only aligns with the right-hand half of Kapelari’s graphs.
Females have only a slightly higher upper reference limit for TSH, and the upper percentiles slope downward very gently with age.
The lower limit, median, and upper limit for TSH are all higher than they are in healthy adults. Although the authors did not provide data on adult intervals on their assay, the TSH interval was generally 0.4 to 4.0 mIU/L in most regions and assays as of 2022.
In contrast, they found these TSH reference intervals in children:
- 0.9 mIU/L for the 2.5th percentile
- 2.3 mIU/L for the 50th percentile (median)
- 5.1 to 5.3 for the 97.5th percentile in girls, and 4.8 to 5.1 for the 97.5th percentile in boys.
This shift upward on all three parameters, even the lower limit, could be the effect of the early morning blood draw in a fasted state. It shows how high “normal” TSH levels can rise in children.
Next, the FT4 reference intervals show a little more variation by age and sex:
For FT4, wobbles appear in puberty but the differences are numerically minimal.
The most interesting anomalies by sex and age are seen in FT3 concentrations.
Why did the FT3 levels shift between boys and girls after the age of 12? The authors commented:
“This difference may reflect puberty-related hormonal changes (e.g. rising estrogen levels), but this study was not designed to investigate this in detail.”(Gunapalasingham et al, 2019)
It is possible that shifts in the relative expression of the three deiodinase enzymes (D1, D2, and D3) is involved in puberty. Female reproductive organs have a significant expression of D3 enzyme (See “Tissue RNA expression of DIO1, DIO2, and DIO3.” The D3 enzyme prioritizes the clearance of T3 hormone to an inactive T2 metabolite slightly more than the conversion of T4 hormone to RT3. It is possible that either the T3 secretion from the thyroid, or T4–T3 conversion rate via D1 and D2 enzyme, do not provide as much circulating T3 to compensate for a faster rate of T3 metabolic losses.
The call for age- and sex-specific reference intervals
Gunapalasingham and team reinforced the inappropriateness of using adult reference values for children:
“The differences demonstrated underline that adult reference values are not applicable to children and that pediatric regional reference values are required.”(Gunapalasingham et al, 2019)
Why are laboratories not providing age-specific intervals?
How many times must researchers call for them?
We already have pregnancy trimester-specific reference intervals, so it cannot be the case that laboratories and physicians are incapable of using age-specific intervals.
This is a foundational tool for the interpretation of TSH screening test results, and for accurately confirming a diagnosis based FT4 and FT3.
TSH-only screening, especially if not based on the correct pediatric reference range, will fail to find cases of dysthyroidism that will compromise early childhood and teenage mental and physical development.
Tradition has established TSH as the sole screening test, but it must be tempered with a hefty dose of skepticism that a normal TSH may be deceptive.
The overreliance on this test was established in the 1980s, 1990s, and early 2000s, before reliable Free T4 and Free T3 tests were widely available.
Certainly, TSH is more reliable than Total T4 and Total T3, because the hypothalamus and pituitary adjusts TSH based on the 0.02 to 0.04% free fraction of T4 and 0.3% free fraction of T3, and the ratio between bound and free can differ widely from person to person.
But now that FT3 and FT4 tests have been available since the mid-2000s, scholars can see how variable the TSH-FT4 relationship, FT4-FT3 relationship, and TSH-FT3 relationship can be.
Scientists now caution that TSH levels can be significantly compromised by severe non-thyroidal illnesses of any type, and even by the time of day of the blood draw. Today, leading thyroid scientists recommend direct free thyroid hormone testing to verify the true thyroid status of the patient (Ling et al, 2018; Sheikh et al, 2018).
In addition, many people forget TSH as an isolated screening test is sensitive to primary thyroid diseases, and a normal result cannot rule out central or peripheral-metabolic HPT axis dysfunctions, nor substances and health conditions that temporarily interfere with TSH secretion rates.
Some cases of primary dysfunction may be cloaked, and worsened, by concurrent central or peripheral metabolism disorders that distort the TSH and make it misleadingly “normal.”
Central hypothyroidism is considered “rare” partly because cases are rarely captured — they are invisible to TSH-only screening systems, and to physicians untrained to look for TSH-FT4 disjoints.
Only certain forms of central hypothyroidism are evident at birth. Others will be acquired during life.
A child may even acquire central hypothyroidism by a head injury during sports, or head trauma from a fall or vehicle accident.
In addition, many physicians fail to diagnose central hypothyroidism when FT4 is low-normal. As mentioned above, a low-normal FT4 result on an immunoassay will often be low on a mass spectrometry assay. A child with a low-normal FT4 and FT3 due to central hypothyroidism is no less hypothyroid than a child with the same low-normal FT4 and FT3 during subclinical primary hypothyroidism.
Cases in which TSH lies about “normal” thyroid status will require age-specific FT4 tests for screening, and age- and sex-specific FT3 to confirm the diagnosis, since an abnormally low FT3:FT4 ratio will reveal whether a nonthyroidal illness may be making TSH unreliable, rather than a physically disabled pituitary (Beck-Peccoz et al, 2017).
Sadly, in many regions of the world, a TSH-reflex or TSH-progressive testing algorithm is in place. When the TSH results are normal, the child or youth will be presumed not to have a primary thyroid disorder, so no thyroid hormones may be tested at all.
Fortunately, such systems often permit a physician to bypass the usual TSH reflex algorithm by giving a good clinical reason. If the child or youth is symptomatic and has biochemical signs of dysthyroidism regardless of a “normal” TSH, a physician may state on the requisition that “central hypothyroidism is suspected” to gain access to a full set of TSH, FT3, and FT4 lab results.
References for all articles cited in the “analyzing normal lab results” series are in a separate post.