Optimisation of thyroxine dose in congenital hypothyroidism

Evaluation of the congenital hypothyroidism screening programme in Iran: a 3-year retrospective cohort study

OBJECTIVE

To evaluate the newborn screening programme for congenital hypothyroidism (CH) in Iran from diagnosis to management and follow-up for 3 years from 2011 to 2014.

DESIGN

Retrospective cohort.

SETTING AND PATIENTS

Seventeen university districts were randomly selected from 30 provinces. Central data in each district were gathered and collectively analysed. Congenital hypothyroid subjects were followed for 3 years.

MAIN OUTCOME MEASURES

Programme coverage, screening and treatment age, recall rate, compliance to follow-ups.

RESULTS

The total number of births in 2011 was 501 726, of which 452 918 neonates (90.3%) were screened and 15 671 (3.46%) were recalled; 1085 (1:462, 0.22%) were confirmed as having CH (57.1%: permanent, 42.9%: transient) and followed for 3 years. Positive predictive value (PPV) for the first screening test was 6.9%. After the second screening, recall rate was reduced to 0.69% and PPV increased to 31.3%. Median age at screening was 6 (3-9) days and for 90.6% of patients treatment was initiated before 40 days of age with a median levothyroxine dosage of 25 µg/day; 131 (13.4%) were lost to follow-up. Mean number of follow-up visits over 3 years was 5.7 (95% CI 5.5 to 5.9) and 23% (n=225) had total compliance to all follow-ups. Median time for thyroid stimulating hormone normalisation was 45 days, 95% CI (41.1 to 48.8).

CONCLUSION

In Iran, despite well-established protocols of screening and detecting CH subjects, stricter implementation of a structured system for monitoring and surveillance is needed to promote the management of patients and to reduce rates of loss to follow-up. Determining and addressing the causes of high false positive rates must be prioritised.

Newborn Screening Guidelines for Congenital Hypothyroidism in India: Recommendations of the Indian Society for Pediatric and Adolescent Endocrinology (ISPAE) - Part II: Imaging, Treatment and Follow-up

The Indian Society for Pediatric and Adolescent Endocrinology has formulated Clinical Practice Guidelines for newborn screening, diagnosis and management of congenital hypothyroidism (CH). This manuscript, part II addresses management and follow-up.

RECOMMENDATIONS

Screening should be done for every newborn using cord blood, or postnatal blood ideally at 48 to 72 h of age. Neonates with screen TSH > 20 mIU/L serum units (or >34 mIU/L for samples taken between 24 and 48 h of age) should be recalled for confirmation. For screen TSH > 40 mIU/L, immediate confirmatory venous T4/FT4 and TSH, and for mildly elevated screen TSH, a second screening TSH at 7 to 10 d of age, should be taken. Preterm and low birth weight infants should undergo screening at 48-72 h age. Sick babies should be screened at least by 7 d of age. Venous confirmatory TSH >20 mIU/L before age 2 wk and >10 mIU/L after age 2 wk, with low T4 (<10 μg/dL) or FT4 (<1.17 ng/dL) indicate primary CH and treatment initiation. Imaging is recommended by radionuclide scintigraphy and ultrasonography after CH is biochemically confirmed but treatment should not be delayed till scans are performed. Levothyroxine is commenced at 10-15 μg/kg in the neonatal period. Serum T4/FT4 is measured at 2 wk and TSH and T4/FT4 at 1 mo, then 2 monthly till 6 mo, 3 monthly from 6 mo-3 y and every 3-6 mo thereafter. Babies with the possibility of transient CH should be re-evaluated at age 3 y, to assess the need for lifelong therapy.

Persistent hyperthyrotropinemia in congenital hypothyroidism: successful combination treatment with levothyroxine and liothyronine

BACKGROUND

Some children with congenital hypothyroidism (CH), have persistent hyperthyrotropinemia despite good compliance with levothyroxine.

OBJECTIVE

To evaluate combination therapy of liothyronine (cytomel) with levothyroxine in CH with persistent hyperthyrotropinemia.

PATIENTS AND METHODS

Files were reviewed retrospectively. Eight female patients with persistently high levels of TSH and upper normal FT4 levels were given either 6.25 or 12 microg liothyronine and the levothyroxine dose was reduced appropriately. Pre- and post-intervention hormone levels and drug doses were evaluated.

RESULTS

TSH decreased in 8/8 and normalized in 6/8 patients. FT4 and free tri-iodothyronine (FT3) remained normal. The levothyroxine-equivalent dose on the combination was 5.0 +/- 0.3 microg/kg/day in infants and 3.4 +/- 0.4 microg/kg/day in children above 2.5 years. Infants required higher liothyronine doses compared with older children (0.66 +/- 0.01 vs. 0.3 +/- 0.05 microg/kg/day).

CONCLUSIONS

Combined therapy can achieve normal TSH levels with normal FT4 and FT3. Further long-term research is required to investigate effects on neurodevelopmental outcome.

Audit of initial management of congenital hypothyroidism in the United Kingdom--comparison of UK practice with European and UK guidelines

BACKGROUND

Prompt and adequate management of newly diagnosed congenital hypothyroidism (CH) has been shown to optimise intellectual outcome.

METHODS

A questionnaire survey of the British Society for Paediatric Endocrinology and Diabetes (BSPED) membership was undertaken, examining current clinical practice in neonatal CH. Results were compared with published management guidelines from Europe and the UK.

RESULTS

The response rate was 86%. The majority were largely compliant with both guidelines. 43% review newly referred infants on the day of notification. However, 26% treat severe CH with < 10 microg/kg/day thyroxine and nearly 20% do not follow up until at least 14 days after initiating treatment, in contrast to both guidelines. Despite a new liquid T4 preparation being licensed, respondents preferred tablet T4.

CONCLUSION

Rapidity of assessment and adequate follow up of suspected CH is critical to outcome. Existing European and UK guidelines should be reviewed and expanded to incorporate new evidence, together with increased advice on preparation and administration of T4.

Ultrasound appearance of thyroid tissue in hypothyroid infants

OBJECTIVE

To identify those infants who need a higher starting dose of levothyroxine (l-T4) for early normalization of thyroid-stimulating hormone (TSH) level.

STUDY DESIGN

TSH levels at 2 time points (1 to 3 weeks and 3 to 5 weeks) after l-T4 therapy at a starting dose of 8 to 12 microg/kg/day were evaluated retrospectively in 22 hypothyroid infants screened for congenital hypothyroidism (CH) in terms of etiology as determined by ultrasonography (US), the size of distal femoral epiphysis (DFE), and initial thyroid function.

RESULTS

The infants with a noneutopic thyroid or small DFE exhibited significantly higher posttherapeutic TSH levels compared with the other infants. Eight of the 9 infants who failed to achieve normalized TSH values at 1 to 3 weeks had noneutopic thyroid. All of the infants with eutopic thyroid exhibited normalized TSH at 3 to 5 weeks, and a significantly greater proportion of the infants with eutopic thyroid exhibited normalized TSH at 1 to 3 weeks compared with those with noneutopic thyroid. Stepwise regression analysis demonstrated that US etiology was a significant independent variable for normalization of TSH at 1 to 3 weeks.

CONCLUSIONS

US examination to identify eutopic or noneutopic thyroid provides useful information for determining the starting dose of l-T4 in hypothyroid infants.

General background on the hypothalamic-pituitary-thyroid (HPT) axis

This article reviews the thyroid system, mainly from a mammalian standpoint. However, the thyroid system is highly conserved among vertebrate species, so the general information on thyroid hormone production and feedback through the hypothalamic-pituitary-thyroid (HPT) axis should be considered for all vertebrates, while species-specific differences are highlighted in the individual articles. This background article begins by outlining the HPT axis with its components and functions. For example, it describes the thyroid gland, its structure and development, how thyroid hormones are synthesized and regulated, the role of iodine in thyroid hormone synthesis, and finally how the thyroid hormones are released from the thyroid gland. It then progresses to detail areas within the thyroid system where disruption could occur or is already known to occur. It describes how thyroid hormone is transported in the serum and into the tissues on a cellular level, and how thyroid hormone is metabolized. There is an in-depth description of the alpha and beta thyroid hormone receptors and their functions, including how they are regulated, and what has been learned from the receptor knockout mouse models. The nongenomic actions of thyroid hormone are also described, such as in glucose uptake, mitochondrial effects, and its role in actin polymerization and vesicular recycling. The article discusses the concept of compensation within the HPT axis and how this fits into the paradigms that exist in thyroid toxicology/endocrinology. There is a section on thyroid hormone and its role in mammalian development: specifically, how it affects brain development when there is disruption to the maternal, the fetal, the newborn (congenital), or the infant thyroid system. Thyroid function during pregnancy is critical to normal development of the fetus, and several spontaneous mutant mouse lines are described that provide research tools to understand the mechanisms of thyroid hormone during mammalian brain development. Overall this article provides a basic understanding of the thyroid system and its components. The complexity of the thyroid system is clearly demonstrated, as are new areas of research on thyroid hormone physiology and thyroid hormone action developing within the field of thyroid endocrinology. This review provides the background necessary to review the current assays and endpoints described in the following articles for rodents, fishes, amphibians, and birds.

Neonatal screening for congenital hypothyroidism in the Netherlands: cognitive and motor outcome at 10 years of age

CONTEXT

Patients with thyroidal congenital hypothyroidism (CH-T) born in The Netherlands in 1981-1982 showed persistent intellectual and motor deficits during childhood and adulthood, despite initiation of T(4) supplementation at a median age of 28 d after birth.

OBJECTIVE

The present study examined whether advancement of treatment initiation to 20 d had resulted in improved cognitive and motor outcome.

DESIGN/SETTING/PATIENTS

In 82 Dutch CH-T patients, born in 1992 to 1993 and treated at a median age of 20 d (mean, 22 d; range, 2-73 d), cognitive and motor outcome was assessed (mean age, 10.5 yr; range, 9.6-11.4 yr). Severity of CH-T was classified according to pretreatment free T(4) concentration.

MAIN OUTCOME MEASURE

Cognitive and motor outcome of the 1992-1993 cohort in comparison to the 1981 to 1982 cohort was the main outcome measure.

RESULTS

Patients with severe CH-T had lower full-scale (93.7), verbal (94.9), and performance (93.9) IQ scores than the normative population (P < 0.05), whereas IQ scores of patients with moderate and mild CH-T were comparable to those of the normative population. In all three severity subgroups, significant motor problems were observed, most pronounced in the severe CH-T group. No correlations were found between starting day of treatment and IQ or motor outcome.

CONCLUSIONS

Essentially, findings from the 1992-1993 cohort were similar to those of the 1981-1982 cohort. Apparently, advancing initiation of T(4) supplementation from 28 to 20 d after birth did not result in improved cognitive or motor outcome in CH-T patients.

Effects of soy protein and soybean isoflavones on thyroid function in healthy adults and hypothyroid patients: a review of the relevant literature

Soy foods are a traditional staple of Asian diets but because of their purported health benefits they have become popular in recent years among non-Asians, especially postmenopausal women. There are many bioactive soybean components that may contribute to the hypothesized health benefits of soy but most attention has focused on the isoflavones, which have both hormonal and nonhormonal properties. However, despite the possible benefits concerns have been expressed that soy may be contraindicated for some subsets of the population. One concern is that soy may adversely affect thyroid function and interfere with the absorption of synthetic thyroid hormone. Thus, the purpose of this review is to evaluate the relevant literature and provide the clinician guidance for advising their patients about the effects of soy on thyroid function. In total, 14 trials (thyroid function was not the primary health outcome in any trial) were identified in which the effects of soy foods or isoflavones on at least one measure of thyroid function was assessed in presumably healthy subjects; eight involved women only, four involved men, and two both men and women. With only one exception, either no effects or only very modest changes were noted in these trials. Thus, collectively the findings provide little evidence that in euthyroid, iodine-replete individuals, soy foods, or isoflavones adversely affect thyroid function. In contrast, some evidence suggests that soy foods, by inhibiting absorption, may increase the dose of thyroid hormone required by hypothyroid patients. However, hypothyroid adults need not avoid soy foods. In addition, there remains a theoretical concern based on in vitro and animal data that in individuals with compromised thyroid function and/or whose iodine intake is marginal soy foods may increase risk of developing clinical hypothyroidism. Therefore, it is important for soy food consumers to make sure their intake of iodine is adequate.

Congenital hypothyroidism: no adverse effects of high dose thyroxine treatment on adult memory, attention, and behaviour

BACKGROUND

In congenital hypothyroidism (CH) it has been questioned whether high dose thyroxine replacement therapy has detrimental effects on memory, attention, and behaviour.

AIMS

To describe memory, attention, and behaviour problems in young adults with CH, and to study possible negative effects of high dose thyroxine replacement therapy.

METHODS

A cohort based follow up study of 49 young adults (mean age 20 years) with early treated CH, and sibling controls (n = 41).

RESULTS

Controlled for age and sex, the CH group attained significantly lower scores than sibling controls on some tests of memory (Wechsler Logical Memory part II: 12.9 versus 17.8; difference 5.2, 95% CI 3.6 to 6.8) and attention (Wechsler Freedom From Distractibility factor: 95.6 versus 104.8; difference 9.9, 95% CI 6.4 to 13.4). They rated themselves with more behaviour problems than did sibling controls (52.7 versus 44.7; difference -7.6, 95% CI -11.2 to -4.0) on the Achenbach Self Report. A high thyroxine starting dose, high serum thyroxine treatment levels during the first six childhood years, and high levels at assessment had no adverse effects on outcome measures at age 20. On the contrary, the results suggest better outcome with higher childhood treatment levels.

CONCLUSIONS

Long term outcome revealed deficits in some aspects of memory, attention, and behaviour in young adults with CH relative to sibling controls. No adverse effects of high dose thyroxine therapy were found on measures of memory, attention, and behaviour problems.

Children with congenital hypothyroidism and their siblings: do they really differ?

OBJECTIVE

Although favorable outcome is typically described in follow-up studies of children with congenital hypothyroidism (CH) identified by newborn screening, IQ reductions and persistent cognitive deficits are still reported. These findings are accounted for by disease and treatment variables as well as methodologic factors including choice of comparison group. Although siblings are ideal because they control for genetic and environmental influences, by definition they have different ages when tested, which can also introduce bias. Because we followed children with CH and their siblings over an extended period of time, there were a number of occasions when both groups were tested at the same age. The purpose of this study was to compare the results of children with CH and their unaffected siblings at the same age and with the same test.

METHODS

The sample consisted of 42 children with CH detected between 1975 and 1985 and their 42 siblings, all of whom were tested with the McCarthy or Wechsler Intelligence Scale for Children-Revised (WISC-R) intelligence tests. Nineteen pairs of children were evaluated at 6 years with the McCarthy, and 30 pairs of children were evaluated at 7 or 9 years with the WISC-R. Recorded for children with CH were disease etiology, bone age and thyroxine levels at diagnosis, age at onset of treatment, and starting dosage of levothyroxine.

RESULTS

Paired t tests revealed that the CH group scored lower than siblings by 8.1 IQ points on the McCarthy and 6.2 points on the WISC-R. Factors contributing to the size of the CH-sibling IQ difference were (1) the etiology of hypothyroidism, reflecting the larger differences by those with athyreosis or an ectopic gland than dyshormonogenesis, and (2) the starting dosage of levothyroxine, with those initially treated with >or=8.2 microg/kg per day having smaller CH-sibling differences than those given lower starting doses. There were no effects of bone age, thyroxine levels at diagnosis, or age at treatment onset.

CONCLUSION

Children with CH treated early in life due to newborn screening may have reduced IQ relative to siblings.

Abnormal cognitive function in treated congenital hypopituitarism

AIMS

To assess cognitive function in school age children with congenital pituitary hormone deficiency (PHD).

METHODS

Ten children with PHD (aged 6.0-15.6 years, mean 11.5 years) and sibling controls (aged 8.7-14.9 years, mean 12.1 years) were assessed using the Wechsler Intelligence Scale for Children (WISC-III UK).

RESULTS

The patients' full scale IQ scores were all below average (mean 75, 95% CI 70-80), but were not significantly different to those of sibling controls (mean 82, 95% CI 75-89). There was no difference in verbal IQ between patients and siblings, but performance IQ was significantly reduced (mean 75, 95% CI 68-82 in patients; mean 88, 95% CI 80-96 in sibling controls). The reduced performance IQ reflected a poorer performance in tasks assessing perceptual organisational skills.

CONCLUSIONS

Data suggest that children with PHD have an IQ that is below average when compared to the population norm and a reduced performance IQ when compared to sibling controls. This may reflect abnormal brain development or could be linked to the impact of hypoglycaemia or low thyroxine concentrations in early life. This information is of value when counselling parents and planning a child's care and education, although further, more extensive studies of patients and siblings are required.

Safety of soy-based infant formulas containing isoflavones: the clinical evidence

Soy protein has been used in infant feeding in the West for nearly 100 y. Soy protein infant formulas have evolved in this interval to become safe and effective alternatives for infants whose nutritional needs are not met with human milk or formulas based on cow's milk. Modern soy formulas meet all nutritional requirements and safety standards of the Infant Formula Act of 1980. They are commonly used in infants with immunoglobulin E-mediated cow's milk allergy (at least 86% effective), lactose intolerance, galactosemia, and as a vegetarian human milk substitute. Largely as a result of research in animal models, concerns have been voiced regarding isoflavones in soy infant formulas in relation to nutritional adequacy, sexual development, neurobehavioral development, immune function, and thyroid disease. We discuss the available clinical evidence regarding each of these issues. Available evidence from adult human and infant populations indicates that dietary isoflavones in soy infant formulas do not adversely affect human growth, development, or reproduction.

Congenital hypothyroidism: developmental outcome in relation to levothyroxine treatment variables

Neonatal screening programs for congenital hypothyroidism (CH) were initiated in the 1970s to ensure early treatment in order to prevent mental retardation. With screening, developmental prognosis is considerably improved, but follow-up studies still report developmental delay compared to controls. To explain the delay, most studies have focused on effects of CH severity, supposedly caused by prenatal hypothyroidism. Nonoptimal treatment could also be an explanatory factor. Treatment guidelines have changed; now, a higher levothyroxine starting dose is recommended. In this review, we first summarize outcome studies in CH versus controls and in mild versus severe CH. Second, we report results on the association between levothyroxine treatment variables and developmental outcome. Six research groups have reported results on the association between levothyroxine starting dose and intelligence, and eight groups on the association between circulating thyroid hormones and intelligence. Most studies found a positive association between a high level of treatment, primarily treatment during the first year and later intelligence. However, negative associations between high-dose treatment and outcome have been reported, and the question of optimal treatment in relation to developmental outcome has not been answered. Effects of high levothyroxine dosage should be documented in samples that have been treated according to recent recommendations.

Congenital hypothyroidism: influence of disease severity and L-thyroxine treatment on intellectual, motor, and school-associated outcomes in young adults

OBJECTIVE

To describe intellectual, motor, and school-associated outcome in young adults with early treated congenital hypothyroidism (CH) and to study the association between long-term outcome and CH variables acting at different points in time during early development (CH severity and early L-thyroxine treatment levels [0-6 years]).

METHODS

Neuropsychological tests were administered to all 49 subjects with CH identified during the first 3 years of the Norwegian neonatal screening program (1979-1981) at a mean age of 20 years and to 41 sibling control subjects (mean age: 21 years).

RESULTS

The CH group attained significantly lower scores than control subjects on intellectual, motor, and school-associated tests (total IQ: 102.4 [standard deviation: 13] vs 111.4 [standard deviation: 13]). Twelve (24%) of the 49 CH subjects had not completed senior high school, in contrast to 6% of the control subjects. CH severity (pretreatment serum thyroxine [T4]) correlated primarily with motor tests, whereas early L-thyroxine treatment levels were related to verbal IQ and school-associated tests. In multiple regression analysis, initial L-thyroxine dose (beta = 0.32) and mean serum T4 level during the second year (beta = 0.48) predicted Verbal IQ, whereas mean serum T4 level during the second year (beta = 0.44) predicted Arithmetic.

CONCLUSIONS

Long-term outcome revealed enduring cognitive and motor deficits in young adults with CH relative to control subjects. Verbal functions and Arithmetic were associated with L-thyroxine treatment variables, suggesting that more optimal treatment might be possible. Motor outcome was associated with CH severity, indicating a prenatal effect.

Suppression of TSH in congenital hypothyroidism is significantly related to serum levels and dosage of thyroxine

AIM

To assess thyrotropin (thyroid-stimulating hormone; TSH) suppression and serum thyroxine (T(4)) concentrations in infants with congenital hypothyroidism in relation to T(4) dose and pretreatment parameters.

METHOD

A retrospective study of all cases treated in a single centre since neonatal screening began was performed.

RESULTS

In 54 infants treated with a mean daily T(4) dose of 9.8 microg/kg, the TSH concentration was suppressed (<6 mU/l) in 65% of the cases by 6 months with the serum T(4) level at the upper end of the infant reference range. Infants who suppressed their TSH later did not differ in pretreatment serum TSH or T(4) concentration. T(4) dose and serum T(4) level were lower in infants whose TSH was not suppressed.

CONCLUSIONS

TSH suppression in congenital hypothyroidism is significantly related to serum levels and dosage of T(4). We suggest that a delay in TSH suppression is mainly due to undertreatment.

Congenital hypothyroidism: a review of current diagnostic and treatment practices in relation to neuropsychologic outcome

Because thyroid hormone is essential for normal brain development, children born with congenital hypothyroidism who lack thyroid hormone during a circumscribed period of early development are at risk of brain damage and mental retardation. Since the advent of newborn screening programs in the 1980s, the diagnosis and treatment of this condition are now provided in the first 2-3 weeks of birth in most regions. While this is usually sufficient to prevent mental retardation, the children so identified attain mildly reduced IQs from expectation, and may still experience subtle and specific neurocognitive deficits. Their particular deficits are related to the brief period of thyroid hormone insufficiency they undergo, especially factors reflecting the severity of hypothyroidism at the time of diagnosis, the duration of hypothyroidism in infancy, and thyroid hormones at time of testing. In this article, we review the specific kinds of deficits demonstrated by children with congenital hypothyroidism who were diagnosed by screening and treated early, as well as the factors associated with their disease and its management that contribute to these deficits. The disease-related factors that will be reviewed will include the etiology of hypothyroidism and severity of disease at the time of diagnosis, while the treatment-related factors will include age at onset of therapy, starting and subsequent dose levels, compliance, and treatment-adequacy issues. Also examined will be the effects of hormone levels at the time of testing. In addition, the role of moderating variables such as social, genetic, and environmental influences, as well as the child's gender, will be discussed. Furthermore, several new issues including the quality of subsequent management, ultimate outcome, and pregnancy will be additionally reviewed. In conclusion, while outcome in congenital hypothyroidism is substantially improved by screening, affected children do still experience mild neuropsychologic deficits. To reduce the impact of persisting deficits, further research is needed to determine the optimal starting dose for the different etiologies, guidelines for subsequent management, and alternative therapies. Moreover, now that the original samples are reaching adulthood and, in females, childbearing age, further research is also needed regarding treatment during pregnancy in women with congenital hypothyroidism, as is research to determine how this population ultimately fares in adulthood.

Dynamics of the plasma concentrations of TSH, FT4 and T3 following thyroxine supplementation in congenital hypothyroidism

OBJECTIVE

The dynamics of the plasma concentrations of various diagnostic determinants of thyroid function were analysed in children with congenital hypothyroidism (CH) after the start of T4 supplementation. The description of the biochemical dynamics of TSH and free T4 (FT4) during the first period of thyroxine treatment is important to depict the practical outlines of the initial dosage of T4 and dosage adjustments for newborns with variable forms of CH.

DESIGN

A retrospective analysis was performed of frequent plasma TSH, total T4 (T4), FT4 and total T3 (T3) measurements in 30 CH neonates during the first weeks of treatment, treated with initial daily T4 dosages ranging from 4.8 to 11.1 microg/kg.

RESULTS

A 50% reduction in the initial plasma TSH concentration was achieved after 3-4 days of treatment, independent of CH severity. At a median of 32 days after the start of T4 supplementation, plasma TSH ranged between 0.4 and 4.0 mU/l. The mean interval needed for FT4 to reach the age-related normal values (12-29 pmol/l) was 3 days. The increase in plasma T3 concentrations levelled off within a few days, when T4 reached concentrations of around 100 nmol/l.

CONCLUSIONS

Plasma T3 and FT4 concentrations reach the normal range a few days after thyroxine treatment is started. By contrast, normalization of plasma TSH concentration takes several weeks. At the time that plasma TSH is normalized, CH neonates show a higher range of plasma FT4 concentrations than the normal range. When TSH normalization is the goal of treatment in CH, the target range for plasma FT4 during treatment in the first months needs to be adapted. During the first month of treatment the plasma TSH concentration is not helpful in assessing the proper T4 supplementation dosage. Once plasma TSH has reached normal values, it becomes a reliable determinant in addition to plasma FT4.