Low-normal concentrations of free thyroxin in serum in late pregnancy: physiological fact, not technical artefact.

Free thyroxin (FT4) concentrations, total thyroxin/thyroxin-binding globulin (T4/TBG) ratios, and thyrotropin (TSH) and albumin concentrations were measured in serum in a longitudinal study in each of the three trimesters of 25 normal pregnancies. In late pregnancy, FT4 estimates by assays reputedly either affected or unaffected by albumin were in the lower half of the reference range for nonpregnant subjects. T4/TBG ratios and albumin concentrations were similarly lower. FT4 overall was significantly (P less than 0.001) correlated with these latter two values. Serum TSH concentrations increased as FT4 declined in late pregnancy. Nonesterified fatty acid (NEFA) concentrations were too low to displace T4 from its binding proteins and were not correlated with other measurements. Within any one of the trimesters, FT4 and T4/TBG were independent of variations in TBG or albumin concentrations. This implies that lower FT4 concentrations in late pregnancy are real, merely coinciding with parallel decreases in albumin. They are not artefacts of albumin-affected assays.

Circadian variation of thyrotropin in childhood.

We evaluated the circadian variation of serum TSH in 96 normal children, aged 5-18 yr. Blood samples were obtained hourly for 24 h, and serum TSH was measured using an immunoradiometric assay with a sensitivity of 0.2 mU/L and an intraassay coefficient of variation of 4.9%. The nadir serum TSH value, defined by the three consecutive hourly TSH concentrations having the lowest mean, occurred between 1000 and 1900 h, while the peak TSH value, defined by the three consecutive hourly TSH concentrations having the greatest mean, occurred between 2100 and 0600 h. The mean nadir serum TSH was 1.6 +/- 0.1 mU/L, and the mean peak TSH was 3.7 +/- 0.2 mU/L. The mean nocturnal TSH surge (percent increase in TSH from nadir to peak) was 144% (95% confidence limits, 50-300%) and did not correlate with serum T4, free T4, or T3 concentrations. Seventy-six children were given TRH (7 micrograms/kg). The mean peak serum TSH after TRH was 16.0 +/- 1.1 mU/L (95% confidence limits, 9.0-42.0 mU/L), and it occurred by 30 min after TRH administration in 92% of the children. The absolute peak nocturnal serum TSH and peak post-TRH serum TSH values correlated significantly (r = 0.62; P less than 0.001), while age, gender, and pubertal status did not correlate with either the nocturnal TSH surge or the TSH response to TRH. We conclude that normal children have a circadian variation of serum TSH characterized by a nocturnal TSH surge, and that the peak of serum TSH, which occurs at night, correlates with the peak serum TSH level after TRH administration.

Euthyroid hyperthyroxinemia due to familial excess of thyroxine-binding globulin.

The correct diagnosis of benign hyperthyroxinemia in this patient and his family members will spare them the unnecessary testing and treatment for thyrotoxicosis that has befallen some such patients. Results of the usual blood tests for assessment of thyroid function, such as T4, T3, and thyrotropin determinations, were not uniformly diagnostic, and were potentially misleading. An increased T4 level, a nonsuppressed TSH level, normal levels of FT4 and FT4D, and a low level of T3RU were clues that led to a request for specific measurement of serum TBG levels in multiple family members; family testing was essential for the diagnosis of euthyroid hyperthyroxinemia due to familial hepatic overproduction of TBG.

Maternal free thyroxine and thyroxine-binding globulin during pregnancy ending in congenital malformations in the offspring.

Thyroxine-binding globulin (TBG) and free thyroxine (FT4) were studied in 172 apparently healthy women giving birth to malformed children. The malformations were divided into two groups: minor malformations (group A) and major malformations (group B). The blood samples were usually drawn in the first trimester. Group A showed no significant difference in TBG values compared to women with healthy children. Group B showed lower, but not significantly lower values in earlier pregnancy. However, from the 15th-16th week of pregnancy the TBG values were significantly lower. The FT4 values were in the hyperthyroid range in as much as 20% of the women, compared to the reported rate of 0.2% hyperthyroidism in pregnant women.

Normal cellular uptake of thyroxine from serum of patients with familial dysalbuminemic hyperthyroxinemia or elevated thyroxine-binding globulin.

To determine whether thyroid hormone-binding proteins in serum, particularly albumin, facilitate the transfer of T4 into human tissues, we studied cellular T4 uptake (CT4) by human liver (Hep G2) cells from medium containing serum from subjects with familial dysalbuminemic hyperthyroxinemia (FDH) and acquired and familial T4-binding globulin (TBG) excess and patients with normal T4-binding to albumin and normal TBG concentrations. Serum from nine subjects with FDH whose mean serum total T4 (TT4) concentration was 203 +/- 27 nmol/L were matched for TT4 concentrations with serum from nine subjects with acquired TBG excess (TT4, 201 +/- 23 nmol/L) and nine subjects with thyrotoxicosis and normal TBG concentrations (TT4, 205 +/- 28 nmol/L). The subjects' CT4 results were compared to their serum free T4 concentration, measured by equilibrium dialysis (DT4), and their serum free T4 index (FT4I) value. The mean serum DT4 value for the subjects with FDH (23 +/- 5 fmol/L) and those with TBG excess (23 +/- 3 fmol/L) were normal, whereas it was elevated (44 +/- 9 fmol/L; P less than 0.001) for the thyrotoxic patients with normal TBG concentrations. The mean CT4 value also was normal for the subjects with FDH (37.7 +/- 4.9 fmol/plate) and those with TBG excess (36.6 +/- 4.6 fmol/plate), but was elevated for the thyrotoxic patients (62.3 +/- 11.2 fmol/plate; P less than 0.001). In all three groups studied, the relationship between individual CT4 and DT4 values was similar to that previously found in subjects with no T4-binding protein abnormalities. The mean serum FT4I value was lower for the subjects with acquired TBG excess (111 +/- 22) than for the subjects with FDH (133 +/- 22; P less than 0.05), and it was much higher for the subjects with thyrotoxicosis (221 +/- 31; P less than 0.001). In the subjects with FDH and those with thyrotoxicosis the normal relationship between CT4 and FT4I was maintained, while in the subjects with acquired TBG excess, FT4I values were lower than expected. In seven of the nine subjects with TBG excess, the abnormality was associated with conditions known to increase its sialic acid content: hepatitis (one subject), pregnancy (four subjects), and estrogen therapy (two subjects). The CT4 values were similar in nine subjects with acquired TBG excess (seven pregnant women and two subjects with chronic active hepatitis) and five subjects with familial TBG excess (34.8 +/- 4.3 vs. 34.0 +/- 8.6 fmol/plate, respectively).(ABSTRACT TRUNCATED AT 400 WORDS)

Detection of the thyroxine-binding globulin (TBG) gene in six unrelated families with complete TBG deficiency.

T4-binding globulin (TBG) is a glycoprotein of hepatic origin which transports thyroid hormone in serum. Inherited TBG defects in man are X-chromosome linked and are expressed in hemizygotes as complete deficiency, partial deficiency, or excess. Since TBG is not necessary for thyroid hormone action, affected subjects are healthy. Using DNA probes for human TBG, we searched for restriction fragment length polymorphisms in six affected males belonging to 6 unrelated families with inherited complete TBG deficiency and an equal number of normal males. TBG could not be detected in the serum of any of the TBG-deficient males by a specific and sensitive RIA capable of detecting as little as 5 micrograms TBG/L or 0.031% of the average normal serum TBG concentration. DNA isolated from white blood cells was digested with 11 restriction endonucleases, and the digests were submitted to DNA blot analysis using two cloned TBG-DNA probes which together covered the entire protein coding and the 5'-flanking sequences of the TBG gene. A total of 26 different bands were detected on DNA blots, identifying 18 restriction sites located within the 4.2-kilobase TBG gene, which includes intronic, exonic, and 5'-flanking sequences. This analysis, which sampled 2.3% of the total TBG genome, failed to reveal differences in fragment size among the 6 TBG-deficient and 6 normal males examined. One restriction endonuclease (NcoI) identified normal sequences at the putative promoter region of the gene, and four other endonucleases (TaqI, SstII, MspI, and HpaII) recognized the cytosine-guanine dinucleotide phosphate sequences representing potential mutation hot spots. Although C was methylated at these sites, no C to T (thymidine) transitions were found. These data suggest that large deletions, insertions, or rearrangements of the TBG gene, or mutations at sites of methylated cytosine-guanine dinucleotide phosphate dimers are not common mechanisms for inherited complete TBG deficiency in man.

The effect of oral testosterone on serum TBG levels in alcoholic cirrhotic men.

Seventy-three euthyroid male patients with alcoholic cirrhosis of the liver were randomly allocated to oral testosterone (200 mg t.i.d.) or placebo and followed for up to 36 months. Triiodothyronine (T3), tetraiodothyronine (T4), thyroxine binding globulin (TBG) and T4/TBG ratio were determined before entry and during follow-up. No significant differences were observed before entry or during follow-up between the two treatment groups. T3, T4 and T4/TBG ratio did not change significantly during follow-up, while TBG concentrations decreased (P less than 0.05). Using a multivariate test, it is demonstrated that testosterone treatment significantly reduced TBG concentrations in cirrhotic men with preserved liver function, like normal men, but not in patients with moderate liver dysfunction. The lack of effect of testosterone in patients with more advanced cirrhosis may be due to a decreased function of sex hormone receptors in their liver.

Value of serum transthyretin measurements in the assessment of marginal protein-energy malnutrition in rats.

The influence of a wide range of protein and/or energy intakes on the serum level of rat transthyretin was studied. Young and adult rats were fed ad libitum diets containing 18, 9, 6, 4 and 0.5% protein (wt/wt) or were fed a control diet in restricted amounts. The transthyretin level was lower in young rats and was normal or slightly higher in adult rats fed low protein diets than in those fed the 18% protein diet. It was decreased with decreasing energy intake in all energy-restricted rats. Moderate energy restriction in rats fed equivalent amounts of protein also lowered the transthyretin level. Rats with similar body weights and similar protein intakes showed marked differences in serum transthyretin level, depending on the amount of energy consumed. Serum transthyretin changes were discussed in relation to the level of transthyretin in cerebrospinal fluid and to the serum concentration of albumin, transferrin and thyroid hormones. The results show that serum transthyretin is more closely related to the protein and energy intakes than to the protein and energy content of the diet. Our results indicate that serum transthyretin measurement is a reliable marker in the detection of early moderate and severe protein-energy restriction.

Thyroid functions in patients with various chronic liver diseases.

In order to clarify an alteration in thyroid functions in patients with chronic liver diseases, serum total and free thyroxine (T4, FT4), total and free triiodothyronine (T3, FT3), total reverse T3 (rT3), thyrotropin (TSH), thyroxine-binding globulin (TBG) concentrations, and T3 uptake (T3U) were measured by radioimmunoassays in 53 patients with chronic hepatitis (CH), 24 patients with compensated liver cirrhosis (LC), 17 patients with hepatocellular carcinoma associated with LC (HCC), and 40 normal subjects. Serum T4, T3, and rT3 in CH, and serum rT3 in HCC were significantly increased, while serum T4 in LC and serum T3 in HCC were significantly decreased. Serum TBG was increased and T3U was decreased in these patients. Serum TBG in CH and LC correlated positively with transaminase, and inversely with prothrombin time. FT4 and T4/TBG ratios in CH and LC and FT3 and T3/TBG ratios in LC and HCC were significantly decreased. Although T4/TBG ratios in HCC and T3/TBG ratios in CH were significantly decreased, FT4 in HCC and FT3 in CH were not decreased. The ratio of rT3/T3 in CH and LC correlated with various liver function tests. FT3 in LC and HCC correlated inversely with BSP (45') and positively with KICG. No differences in serum TSH values were found between chronic liver diseases and normal subjects. From these results, it was concluded that the thyroid functions in patients with chronic liver diseases were affected by the decrease in serum thyroxine, elevated serum TBG, the degree of which is in proportion to that of the liver cell damage, and impaired peripheral conversion of T4 to T3, the degree of which is in proportion to that of the hepatic dysfunction.

Relationship of oligosaccharide modification to the cause of serum thyroxine-binding globulin excess.

T4-binding globulin (TBG), a glycoprotein with four N-glycosyl complex oligosaccharide chains, exhibits sialic acid-dependent microheterogeneity on isoelectric focusing (IEF). Increasing the sialic acid content of TBG increases its anodal IEF mobility and decreases its rate of in vivo degradation, a mechanism for the elevation of serum TBG levels in pregnancy. In this study, the structure of oligosaccharides in TBG from subjects with various conditions associated with TBG excess was determined by measuring the proportion that bound to Concanavalin-A (Con-A). Since oligosaccharides with three or more branches (antennae) attached to the trimannosyl core are excluded from binding to Con-A, the percentage of serum TBG not bound to Con-A (% peak A) represented the portion of TBG molecules with three or more antennae in all oligosaccharide chains interacting with Con-A. Peak A contained the most anodal IEF bands, while the Con-A-bound TBG (peak B) contained the cathodal bands. Serum samples from 10 normal men and 10 premenopausal women did not significantly differ in terms of TBG levels, % peak A, or IEF mobility and were combined as a single group (normal). Eight subjects with elevated serum TBG levels due to inherited TBG excess [62.0 +/- 10.1 (+/- SD) mg/L] or 2 receiving 5-fluorouracil treatment (26.2 and 31.3 mg/L) compared to 20 normal (14.7 +/- 3.3 mg/L) had % peak A values and IEF mobility similar to those in normal subjects. On the other hand, high serum TBG levels in 8 women during the third trimester of pregnancy (39.2 +/- 5.3 mg/L), 2 women taking oral contraceptives (25.7 and 27.0 mg/L), and 3 women with acute hepatitis (34.8 +/- 4.8 mg/L) were associated with significant elevations of % peak A values (5.68 +/- 1.73%, 3.31% and 2.41%, and 3.25 +/- 0.78%, respectively) compared to those in normal subjects (1.33 +/- 0.40%), as well as increased anodal mobility on IEF. Treatment of a man for 3 days with ethinyl estradiol produced similar changes. Using data from densitometry measurements of IEF bands of TBG, the degree of anodal shift was quantitated (anodal index). This index correlated with the % peak A (r = 0.92) in all study subjects. We conclude that increased sites for sialylation, resulting from the increased proportions of triantennary oligosaccharide chains, account for the increased anodal mobility of TBG in hyperestrogenemia and hepatitis. Thus, in these two conditions, a reduced TBG degradation rate resulting from oligosaccharide modification is the likely mechanism of increased serum TBG levels.(ABSTRACT TRUNCATED AT 400 WORDS)

Differential binding of thyroxine and triiodothyronine to acidic isoforms of thyroid hormone binding globulin in human serum.

The differential availability of thyroxine (T4) and 3,5,3'-triiodothyronine (T3) to liver from the circulating thyroid hormone binding globulin (TBG)-bound pool suggests that the two thyroid hormones may bind to different TBG isoforms in human serum. In the present study, the binding of [125I]T4 and [125I]T3 to human serum proteins was investigated by using slab gel isoelectric focusing and chromatofocusing. In normal human male serum, [125I]T4 was localized to four isoforms of TBG called TBG-I, -II, -III, and -IV, with isoelectric points (pI's) of 4.30, 4.35, 4.45, and 4.55, respectively. [125I]T3 was localized to only two isoforms of TBG, TBG-III and -IV, with pI's that were identical with those for [125I]T4. In normal female serum, [125I]T4 was localized to the same four isoforms of TBG as those of normal male serum, while [125I]T3 was localized to TBG-II, -III, -IV, and -V (pI = 4.65). In pregnant female serum, [125I]T4 was localized to five isoforms, whereas [125I]T3 was localized to four. IEF was also performed with male serum loaded with various concentrations of unlabeled T3. The Ki values of T3 binding to TBG-I, -II, -III, and -IV were 5.0, 2.4, 0.86, and 0.46 nM, respectively. The TBG isoforms in normal male serum were also separated by sequential concanavalin A-Sepharose affinity chromatography and chromatofocusing (pH range of 3.5-5.0). T4 preferentially bound to the most acidic isoforms of TBG in the pI range of 3.8-4.0, whereas the less acidic fractions (pH 4.0-4.2) bound both T4 and T3. In conclusion, this study shows that T4 and T3 do not bind to a single competitive binding site on TBG. Instead, T4 is preferentially bound by the most acidic TBG isoforms owing to a 10-fold lower affinity of T3 for these proteins.

Effect of infection with Spirometra erinacei plerocercoid on thyroid hormone in mice.

Young, male ICR mice were given tap water or distilled water containing 200 mg/l propylthiouracil (PTU) and were then infected with 10 plerocercoids of Spirometra erinacei to investigate the effect of plerocercoid infection on thyroid hormone in their hosts. Plerocercoid infection stimulated growth in PTU-induced hypothyroid mice as if they had never received PTU treatment: there were increases in weight in the liver, skeletal muscle, and spleen, as well as enhancement of the head and body length, in spite of a greater decrease in serum T4 levels than was observed in PTU-treated controls. Furthermore, the intact mice infected with plerocercoids showed a decrease in serum T4 levels as well as in the concentration of T4-binding globulin. These observations suggest that the growth stimulation and the decrease in concentrations of serum T4 and T4-binding globulin associated with plerocercoid infection in mice probably resulted from secretion of a growth hormone-like substance produced by plerocercoids of S. erinacei.

Serum thyroid hormones, thyrotropin and thyroxine binding globulin during prolonged strength training.

The effects of progressive strength training for 24 weeks on maximal strength and pituitary-thyroid function were studied in 21 males during the training and during the following detraining period of 12 weeks. Maximal strength increased greatly (p less than 0.001) in the first 20 weeks, followed by a plateau phase in the last 4 weeks of training. Maximal strength decreased greatly (p less than 0.001) during the detraining period. The concentrations of serum total (T4) and free thyroxine (fT4) decreased (p less than 0.05 and less than 0.01, respectively) during the training period and they rose to pretraining levels during the detraining period. During the most intense training phase (the last 4 weeks) there was a positive correlation between the changes in serum fT4 concentrations and the changes in maximal force (r = 0.56; p less than 0.01). No statistically significant changes occurred in the levels of serum triiodothyronine, thyrotropin or thyroxine binding globulin. The results show that prolonged intensified strength training can slightly decrease the concentrations of serum total and free T4. These small changes cannot have any clinical significance, and even their physiological significance may be only marginal.

Thyroid hormones and thyroxine-binding globulin in relation to liver function and serum testosterone in men with alcoholic cirrhosis.

In 73 euthyroid male patients with histologically verified alcoholic cirrhosis, thyroid hormones, thyroxine-binding globulin (TBG) and testosterone concentrations (total, non-protein- and non-SHBG-bound) were studied in relation to each other and to the degree of liver dysfunction. Serum concentrations of triiodothyronine (T3) decreased significantly (p less than 0.05) and thyroid-stimulating hormone (TSH) increased with progressing liver dysfunction. Serum concentrations of tetraiodothyronine (T4), TBG and T4/TBG ratio did not correlate significantly with liver function. Serum T3 concentrations correlated significantly (Kendall Tau-beta = -0.33, p = 0.001) with total serum testosterone concentrations, while there was a negative correlation (Kendall Tau-beta = -0.20, p = 0.025) between testosterone and TSH values. No correlation was found between testosterone concentrations and serum levels of TBG. It is proposed that the association between T3 and TSH on one hand and testosterone concentrations on the other reflects a covariation of these variables with liver function. The TBG level was normal in most patients and was not correlated to testosterone concentrations.

Seasonal changes in serum thyroid hormone binding proteins in the woodchuck (Marmota monax).

To evaluate the role of serum T4- and T3-binding proteins in the elevation of serum T4 and T3 concentrations in the woodchuck in the fall and winter, blood was collected from woodchucks during the four seasons of the year (seasonal study) and from 2-week fasted woodchucks in the summer (fasting study) and the serum concentrations of total T4 and T3 were measured. The distribution of [125I]4 and [125I]T3 tracers among the serum binding proteins and the serum T4-binding globulin (TBG) binding capacity for T4 was determined by polyacrylamide gel electrophoresis. Plasma concentrations of both T4 and T3 were highest in the winter. The major T4-binding protein in the woodchuck is TBG. There was an increase in both [125I]T4 and [125I]T3 tracer binding to serum TBG in fall and winter, and TBG binding capacity for T4 was 2-fold higher in winter than in summer. There were increases in TBG binding and in the TBG T4 binding capacity in the 2-week starved animals. The increased binding of T4 and T3 by TBG in the fall and winter may be partially responsible for the increased serum concentrations of T4 and T3 in the fall and winter in the woodchuck, a time when secretion of T4 and T3 by the thyroid gland is very low. This may be facilitated by the low or absent food consumption at these times of the year.

Serum thyroid hormone binding inhibitor in nonthyroidal illnesses.

We have employed the recently developed competitive ligand binding assay (CLBA) to study thyroid hormone binding inhibitor (THBI) in ether extracts of sera of 25 patients admitted to the Medical Intensive Care Unit with a variety of nonthyroidal illnesses (NTI). THBI was detected in 60% (15/25) of patients using one sample/patient and in 88% (15/17) using multiple (two to six) samples from different days. Mortality rate and mean serum concentrations of total T4, total T3, and albumin were similar in THBI-positive and THBI-negative patients. There was a tendency for a higher frequency of low serum total T4 in THBI-positive (10/15) than in THBI-negative (3/10) patients but the difference was not statistically significant (P less than 0.1 by Chi square). However, the mean dialyzable fraction of T4 (DFT4 0.11 +/- 0.02%, n = 9 v 0.054 +/- 0.004%, n = 10) and DFT3 (0.54 +/- 0.05% v 0.40 +/- 0.032%) were both significantly (P less than 0.05) higher in THBI-positive patients than THBI-negative patients. There was a significant correlation between THBI and DFT4 (r = 0.55, P less than 0.02) or DFT3 (r = 0.54, P less than 0.02). Prior extraction of serum with ether reduced DFT4 in NTI patients with high baseline DFT4 but not in normal subjects or NTI patients with mildly abnormal baseline DFT4. Addition to a normal serum (0.1 mL) of evaporated ether extract of a pooled NTI serum (0.10- to 3.0-mL equivalent) increased DFT4 progressively from 0.025% to 0.14%. Similar extract of a pooled serum of normal subjects had little or no effect.(ABSTRACT TRUNCATED AT 250 WORDS)