Use of direct thyroxine-binding globulin measurement in the evaluation of thyroid function

The clinical use of thyroid function tests

Laboratory tests are essential for accurate diagnosis and cost-effective management of thyroid disorders. When the clinical suspicion is strong, hormonal levels just confirms the diagnosis. However, in most patients, symptoms are subtle and unspecific, so that only biochemical tests can detect the disorder. The objective of this article is to do a critical analysis of the appropriate use of the most important thyroid function tests, including serum concentrations of thyrotropin (TSH), thyroid hormones and antithyroid antibodies. Through a survey in the MedLine database, we discuss the major pitfalls and interferences related to daily use of these tests and recommendations are presented to optimize the use of these diagnostic tools in clinical practice.

Interval between tests and thyroxine estimation method influence outcome of monitoring of subclinical hypothyroidism

CONTEXT/OBJECTIVE

Most patients with subclinical hypothyroidism are regularly monitored when treatment is not started. We have studied how interval between follow-up visits and how different T(4) estimates influence diagnostic outcome in a cohort of patients with untreated subclinical hypothyroidism, and studied whether assessment of clinical symptoms and signs aids evaluation of an individual subclinical hypothyroidism patient.

DESIGN/PATIENTS

During 1 yr, monthly measurements of TSH and three different T(4) estimates, and recording of hypothyroid symptoms and signs were performed in 21 patients with subclinical hypothyroidism confirmed on two occasions 3 months apart.

RESULTS

One patient was euthyroid at all visits, and one started treatment for profound overt hypothyroidism. The remaining patients were subclinical hypothyroidism at 74%, overtly hypothyroid at 22%, and had normal thyroid function tests in 4% of the visits. Increasing frequency of visits associated significantly with decreasing number of patients characterized as subclinical hypothyroidism after 1 yr (P = 0.016). Diagnosis of overt hypothyroidism differed between T(4) estimates (P = 0.005) and was highly dependent on T(4) reference limits. The hypothyroid clinical score did not differ between biochemical diagnoses (P = 0.29).

CONCLUSIONS

The monitoring procedure itself may influence the outcome of control of subclinical hypothyroidism. Specifically, the interval between visits, type of T(4) estimate used, and lower T(4) reference limit influenced the outcome when untreated subclinical hypothyroidism patients were followed for 1 yr. The hypothyroid clinical score did not aid the evaluation in individual subclinical hypothyroidism patients.

Goiter and pregnancy: a new insight into an old problem

Evidence is presented that pregnancy constitutes a goitrogenic stimulus, particularly in conditions with a restricted or even a marginally low iodine intake. In a series of studies carried out in a large cohort of pregnancies in the Brussels area, the authors show that an increase in thyroid volume is observed in a majority of pregnant women, leading to goiter formation at delivery in 9% of the cases. Furthermore, increments in thyroid volume were correlated with biochemical evidences of functional stimulation of the thyroid, such as an elevation in serum TG levels, preferential T3 secretion, and slight increases in basal TSH at delivery. Hence, the association of biochemical features of thyroidal stimulation with volumetric changes in the gland strongly suggests that pregnancy truly induces goitrogenesis rather than vascular swelling ("intumescence") alone, at least in conditions with a low iodine intake. Finally, preliminary data from this laboratory, as well as recently published data from other investigators, suggest that goiter formation during pregnancy can easily be prevented by increasing the iodine supply during pregnancy.

Dietary restriction on serum thyroid hormone levels

To determine the long term effects of a protein sparing fast on serum thyroid hormone levels, the authors studied 38 obese patients ingesting a diet of 320 kcal for up to 13 weeks. The high baseline serum triiodothyronine (T3) levels decreased significantly by the first week, further decreased by the third week, and this lower level persisted for the duration of the fast until realimentation. Serum free T3 index followed the same general pattern as did serum T3 levels. Serum reverse T3 increased significantly by the first week, but by week three, the reverse T3 level had begun to fall, although still significantly increased above baseline. By week seven, reverse T3 had decreased to almost baseline and remained not significantly changed from the baseline to 13 weeks. Serum thyroxine (T4) increased significantly by the first week in all patients, but by the third week had returned to baseline levels which persisted to 13 weeks. The free T4 index and free T4 concentrations showed the same increment at week one and then returned to baseline levels. There were no significant changes in serum thyroxine-binding globulin (TBG) or thyroid-stimulating hormone (TSH) concentrations. The changes in serum T3 and reverse T3 levels are attributable to alterations in peripheral 5'-monodeiodination of T4 and reverse T3 induced by the protein sparing fast.

Triiodothyronine uptake measurement in serum by time-resolved fluorescence immunofluorometry

A solid phase competition-type fluoroimmunoassay for triiodothyronine (T3) uptake in serum is described. In the assay, exogenous free T3 binds to the unoccupied binding sites on serum thyroxine binding proteins while the remaining unbound T3 competes with immobilized T3 for binding to a soluble biotinylated anti-T3 monoclonal antibody. The bound biotinylated antibody is quantitated by the addition of streptavidin labeled with the europium chelator 4,7-bis(chlorosulfophenyl-1,10 phenanthroline-2,9-bicarboxylic acid (BCPDA) in the presence of excess europium. The fluorescence signal of the final complex, which is directly proportional to the number of unoccupied binding sites on thyroxine binding proteins, is then measured on the dried solid-phase with a pulsed-laser time-resolved fluorometer. The assay requires a 10 microliters serum sample and a total incubation time of 90 minutes. The coefficients of variation for within-run and between-run assays ranged from 2.0 to 5.7%. Results obtained by the present method compared well with those determined by two commercial radioimmunoassays (r greater than 0.9).

Time-resolved immunofluorometric assay for thyroxine-binding globulin in serum

We describe a new "sandwich"-type immunofluorometric assay for thyroxine-binding globulin (TBG) in serum. The assay involves a solid-phase monoclonal antibody immobilised in white microtiter wells, and a soluble biotinylated monoclonal antibody that reacts with the captured TBG molecules. Addition of streptavidin labeled with the europium chelator, BCPDA (4,7-bis(chlorosulfophenyl)-1,10 phenanthroline-2,9-dicarboxylic acid), and excess europium results in the formation of a highly fluorescent product. The fluorescence signal of the final complex is quantitated on the dried solid-phase with a pulsed-laser time-resolved fluorometer. The assay requires a 151-fold sample pre-dilution and a total incubation time of 90 minutes. It has a broad dynamic range of 0-100 mg/L and a minimum detection limit of 0.4 mg/L. The coefficients of variation for within-run and between-run assays averaged 4.5% and 5.4%, respectively. The mean analytical recovery of TBG added to serum was 103%. Results obtained by this method correlated well with those determined by a commercial radioimmunoassay (r = 0.96, n = 112) and by an immunoradiometric procedure (r = 0.95, n = 131).

Comparative study of neurological and myxoedematous cretinism associated with severe iodine deficiency

The clinical and biochemical features of myxoedematous and neurological cretinism were studied in an endemic goitre area in Algeria (goitre prevalence 51.3%; endemic cretinism 1.1%; mean urinary iodine level 127.6 nmol/l). When comparing the data collected in six different villages of the area, significant negative correlations were found between the decrease in urinary iodide and iodide/thiocyanate ratio (I/SCN) and the increase in prevalences of visible goitres, endemic cretinism and transient neonatal hypothyroidism. The cretins were divided on clinical signs into two groups: myxoedematous (MC) and neurological (NC) cretinism. Differences were noted in the neurological signs and the type of deafness encountered in both types. Although some overlap did exist, proximal spasticity and rigidity were characteristic of NC. The hormonal profiles of the two groups, including TRH tests, were clearly different. The two groups were similar with regard to the percentage of palpable thyroids, the absence of antimicrosomal and anti-thyroglobulin antibodies, seropositive viral antibodies and thiocyanate concentrations in serum and urine. Thus it is unlikely that these factors have any significant aetiological role in NC. The data collected in the general population in this area and those obtained in the mothers of the myxoedematous and neurological cretins support the hypothesis that the neurological signs are the result of hypothyroxinaemia in the mothers and the fetus at different periods of pregnancy. They could be aggravated by neonatal hypothyroidism, which may be transient in NC and permanent in MC.

Biologic effects of transdermal estradiol

We conducted a dose-response study in 23 postmenopausal women to compare the physiologic effects of transdermal estradiol and oral conjugated equine estrogens. The doses studied were 25, 50, 100, and 200 micrograms of transdermal estradiol per 24 hours, and 0.625 and 1.25 mg of oral conjugated estrogens. Transdermal estradiol increased circulating concentrations of estradiol and estrone. Oral conjugated estrogens also raised the levels of estrogen, particularly estrone. Both preparations lowered gonadotropin levels, decreased the percentages of vaginal parabasal cells, increased the percentage of superficial cells, and lowered urinary calcium excretion. The effects of 0.625 and 1.25 mg of oral estrogens were similar to those of 50 and 100 micrograms of transdermal estradiol per 24 hours, respectively. Oral estrogens significantly increased circulating levels of renin substrate, sex-hormone-binding globulin, thyroxine-binding globulin, and cortisol-binding globulin; transdermal estradiol had no effect. The higher dose of oral estrogens had favorable effects on concentrations of low-density and high-density lipoproteins, but transdermal estradiol did not. Neither preparation affected any of the four clotting factors studied. These data indicate that transdermal estradiol can elicit many of the desirable actions of estrogen while avoiding the pharmacologic effects of oral estrogens on hepatic proteins.

Increased risk of primary hypothyroidism in preterm infants

Serum levels of thyroid stimulating hormone, thyroxine, triiodothyronine, free T4, thyroxine-binding globulin, reverse T3, and the TSH secretory areas and peak T3 after intravenous injection of 40 micrograms thyrotropin-releasing hormone were determined weekly from day 5 to 6 to 11 weeks of age in 42 unselected full-term and 61 preterm Belgian infants. The results on day 5 indicated a progressive deficit of thyroid function related to the degree of prematurity. In 92 infants this deficit progressively decreased with age and disappeared at 5 to 7 weeks. However, 11 infants developed biochemical evidence of overt but transient hypothyroidism. Belgian neonates are relatively iodine deficient, and this factor affects the constitution of iodine stores within the thyroid gland: (1) the urinary concentrations of iodine in the 103 infants studied in Belgium were markedly lower than in 30 infants from California; and (2) The iodine concentration of the thyroid gland in preterm infants who died during the 10 first days of life was almost three times lower in Brussels than in Toronto. The results indicate that, in Belgium, the effects of relative iodine deficiency on thyroid function are superimposed on and mask the physiologic state of tertiary hypothyroidism in prematurity.

Determination of free thyroid hormones and their binding proteins in a patient with severe hyperthyroidism (thyroid storm?) and thyroid encephalopathy

A patient with severe hyperthyroidism (thyroid storm?) and thyroid encephalopathy is described. During her illness only a slightly raised level of total thyroxine and a normal level of total triiodothyronine was found in contrast with very high levels of free thyroid hormones. Very low levels of thyroxine binding globulin, albumin and low levels of thyroxine binding prealbumin in contrast with nearly normal values of T3 resin uptake were observed. All parameters of thyroid function returned to normal after therapy.

Endemic infantile hypothyroidism in a severe endemic goitre area of central Africa

Thyroid function and exposure to dietary goitrogenic factors (iodine deficiency and thiocyanate overload) were studied at birth and from birth to 7 years in 200 neonates and 347 children living in the severe endemic goitre area of Ubangi, Northern Zaire. Serum T4 was at the lower limit of normal at birth (104 +/- 4 nmol/l) and stayed at that level during the first year of life (123 +/- 9) (NS), but decreased to 75 +/- 8 (P less than 0.001) at 2-4 years and to 62 +/- 6 (P less than 0.001) at 5-7 years of age. Mean serum FT4 decreased from 10.4 +/- 0.9 pmol/l during the first year to 8.2 +/- 1.0 (NS) at 2-4 years (NS) and to 7.7 +/- 0.9 (P less than 0.05) at 5-7 years. Mean serum TSH was 10.4 (8.4-12.9) mU/l (geometric mean +/- 1 SEM) during the first year, 10.1 (7.5-13.7) (NS) at 2-4 years and 24.3 (18.5-31.9) (P less than 0.05) at 5-7 years. Mean serum T3 was 3.23 +/- 0.12 nmol/l during the first year and remained stable thereafter. The frequencies of low T4 (T4 less than 77 nmol/l), high TSH TSH (TSH greater than 50 mU/l), and low T4 and T3 (T3 less than 1.69 nmol/l) were twice as high at 5-7 years as in the first year (respectively 65%, 42% and 15%). The urinary iodide concentration of the children was stable and low throughout the study period. By contrast, serum thiocyanate concentration which was high at birth (129 +/- 5 mumol/l) decreased to normal values between 3 and 12 months of age and increased again during and after weaning (1 to 3 years of age) to reach a value of 138 mumol/l which was comparable to that observed in adults in the same area. Thiocyanate concentration was high (133 +/- 7 mumol/l) in the mothers' serum but low in the mothers' milk (57 +/- 3 mumol/l) (P less than 0.001). Multivariate analysis showed that both iodine deficiency and thiocyanate overload were explanatory factors of the serum levels of T4, FT4 and TSH in children. In conclusion, our results show that infantile hypothyroidism is much more frequent at 5-7 years of age than at birth or during the first year of life. The deterioration in thyroid function during and after weaning is linked to persistent iodine deficiency accompanied by an increase in thiocyanate overload. The variability in the age of onset, the severity, and the duration of infantile hypothyroidism might explain the wide range of psychomotor and physical abnormalities observed in a large proportion of subjects in this area.

Thyroxine and thyroxine-binding globulin: discriminant formulae for the separation of the diagnostic classes of thyroid function

The biochemical classification of thyroid functional status is often accomplished by the use of the thyroxine to thyroxine-binding globulin (T4:TBG) ratio. It has recently been shown, however, that the reference range of the T4:TBG ratio varies with the concentration of TBG. This makes the T4:TBG ratio an unwieldy diagnostic index. A more accurate and economical way to define thyroid function using T4 and TBG measurements is by linear discriminant formulae. This is so because T4 varies linearly with TBG at the T4 reference limits over a wide range of TBG concentrations. Using the T4 reference ranges established by Attwood and Atkin (Ann Clin Biochem 1982; 19: 101-3), the following formulae are obtained: T4 (nmol/l)--4.05 TBG (mg/l) greater than 94.5 indicates hyperthyroidism; T4 (nmol/l)--2.72 TBG (mg/l) less than 23.8 identifies hypothyroidism.

Estrogen replacement therapy by transdermal estradiol administration

To determine whether the nonoral administration of estradiol (E2) might provide physiologic replacement without alteration of hepatic function, 20 postmenopausal women were studied before and after 3 weeks of treatment with either E2-containing transdermal therapeutic systems or placebo. Twenty premenopausal women were also studied. With E2-containing systems, serum E2 and estrone levels were restored to the premenopausal range. Variable responses of the different biochemical and biologic markers of the actions of E2 were observed. The most sensitive marker was vaginal cytology, with the E2 dosage reverting the maturation index to premenopausal values. Hot flashes, measured objectively, were reduced in frequency but not abolished. Serum levels of follicle-stimulating hormone and luteinizing hormone were lowered but remained higher than the premenopausal range. No significant changes were noted in urinary calcium/creatinine and hydroxyproline/creatinine ratios, which were used as markers of bone resorption. With active systems, no significant changes were noted in the concentrations of the hepatic proteins renin substrate and thyroxine-binding globulin or in the binding capacities of cortisol-binding globulin and sex hormone-binding globulin. These results indicate that transdermal E2 administration may be used to provide estrogen replacement while exerting limited effects on hepatic function.

Comparison of human thyroxine-binding globulin purification by affinity chromatography procedures

Three procedures for the isolation of thyroxine-binding globulin from human serum, using affinity chromatography on triiodothyronine (T3) linked to Sepharose (A), thyroxine (T4) linked to Sepharose (B) or T3 linked to epoxy-Sepharose (C) as the first purification step, were compared. With the use of additional purification steps, the three procedures yielded pure thyroxine-binding globulin without desialylation. With procedure A, the initial binding of T4-binding globulin to T3-Sepharose was very low, yielding a poor final recovery (17%). Procedure B gave the highest yield (35%) after a three-step purification, with a low T4 content (0.15-0.30 mol/mol). Procedure C also gave a high yield (28%) after only two purification steps, with a T4 content greater than 0.7 mol/mol. The microheterogeneity of T4-binding globulin obtained with these three procedures was demonstrated by isoelectric focusing: five major bands were observed between pH 4.1 and 4.6, and intermediate faint bands (often doublets) in the same pH range. However, with procedures A and C, the most acidic bands (pH 4.10-4.20) were always absent. Thyroxine-binding globulin was preincubated with radioactively labelled T3 or T4 and the hormone-protein complex was analyzed by isoelectric focusing. The binding of T3--compared to that of T4--was reduced in the most acidic protein subspecies. These results suggest differences in the thyroid hormone binding properties of the various subspecies of human T4-binding globulin.

Pharmacological interference with in vitro tests of thyroid function

Numerous drugs may cause changes in the serum concentrations of T4 and of T3. If such alterations are not recognized an incorrect diagnosis may result. In moderate degrees of hypo- and hyperthyroidism thyroid hormone levels may be spuriously normal, or the influence of pharmacological substances may lead to false diagnosis of thyroid disease in euthyroid patients. Since prediction of such alterations remains uncertain, it may be necessary to perform additional investigations when a potential artefact is recognized. On the other hand many pharmacological agents, especially those which interact with neurotransmitters, may influence TSH secretion, too. The TRH-test may show an increase or decreased TSH response, although complete suppression is only rarely seen during high-dose glucocorticoid treatment when low TRH doses are applied. Because of TRH-test gives such wide separation between different clinical states false interpretations are generally less likely than with drug-induced changes in T4 and T3 values.

Concentration of serum thyroid hormone binding proteins after 131I treatment of hyperthyroidism

Serum concentrations of the thyroid hormone binding proteins, thyroxine binding globulin, prealbumin, and albumin were determined in 30 thyrotoxic patients before and after 131I treatment. Each patient was placed into one of three groups according to response to treatment. The serum concentration of all three proteins rose significantly in 10 patients who became euthyroid, and a greater increase was seen in 10 patients who developed hypothyroidism. There was no significant change in thyroid hormone binding protein concentrations in 10 subjects who remained hyperthyroid. Changes in the concentration of thyroid hormone binding proteins should be borne in mind when total thyroid hormone concentrations are used to monitor the progress of patients receiving treatment for hyperthyroidism.

Alterations in circulating thyroid hormones and thyroxine binding globulin in chronic alcoholism

The present study concerns eighteen chronic alcoholics with minimal liver damage. A significant reduction in total serum T4 with an accompanying drop in circulating TBG was observed in these otherwise euthyroid patients. During alcohol withdrawal, we observed a rapid increase in T4 and TBG into the normal range. We suggest that the fall in the level of circulating thyroxine-binding globulin is related to a defect in its synthesis or secretion by the liver due to ethanol consumption. Caution is recommended in the interpretation of thyroid function tests in chronic alcoholics.

Alterations in circulating thyroid hormones and thyroxine-binding globulin levels during diabetic ketoacidosis

During diabetic ketoacidosis, in 17 adult patients, significant decreases in serum TBG and total T4 levels were observed, without significant alteration of the T4 to TBG binding property. In addition, serum free T4 (FT4) was moderately elevated and the TSH response to TRH was markedly blunted. No correlation, however, was found between TSH blunting and FT4 elevation. Correlation of these serum anomalies required at least 5 days of adequate control of the diabetes. Thus, diabetic ketoacidosis in euthyroid patients is characterized by multiple alterations in thyroid function parameters and caution is recommended in the interpretation of thyroid tests during and in the days following this severe metabolic disorder.