Subclinical hypothyroidism and hyperlipoproteinaemia: indiscriminate L-thyroxine treatment not justified

It is still under discussion whether subclinical hypothyroidism is a biochemical syndrome or a disease associated with an increased risk for development of vascular diseases due to lipid elevation. Therefore, we investigated lipid values in 40 patients with subclinical hypothyroidism, which is defined in terms of normal (N = 26) or slightly increased (N = 14) basal TSH values and/or an exaggerated TSH response (N = 34) to TRH (> 25 mU/l). Patients with increased lipid values were treated with L-thyroxine and reanalysed three months later. Mean levels of total cholesterol. LDL- and HDL-cholesterol and triglycerides in patients with subclinical hypothyroidism were comparable with those in normal subjects. Individual analysis, however, revealed hyperlipoproteinaemia (HL) in 22.5% of the patients investigated (HL type IIa in seven, type IV in two patients). Thyroid function was the same in affected patients as in those with normal lipid values, whereas higher age was significantly more often associated with this syndrome (p < 0.01). Treatment with L-thyroxine resulted in a significant decrease in total and LDL-cholesterol (p < 0.05), although a normalization of their lipid values could be obtained only in half of the patients. None of the subjects with hyperlipoproteinaemia had a history or clinical signs of actual vascular disease. Although the incidence of hyperlipoproteinaemia in our study group of patients with mild subclinical hypothyroidism (22.5%) is comparable to that of the normal population (21.5%), it is more severe in the former group (LDL-cholesterol in patients 5.26 +/- 0.58 vs 4.8 +/- 0.56 mmol/l in controls; p < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Thyroxine prescription in the community: serum thyroid stimulating hormone level assays as an indicator of undertreatment or overtreatment

Examination of thyroxine usage in a study in the United States of America revealed that many patients were prescribed thyroxine for non-thyroid indications, such as obesity and fatigue. Many of those receiving thyroxine had high or low serum thyroid stimulating hormone levels, indicating prescription of incorrect doses or lack of patient compliance with therapy. Long term thyroxine therapy may have effects upon the risk of osteoporosis. The aims of this study were to investigate indications for thyroxine prescription in the United Kingdom and to examine the frequency of abnormal serum thyroid stimulating hormone concentrations in those prescribed thyroxine for hypothyroidism. This was in order to determine the relevance of measurement of thyroid stimulating hormone level in monitoring thyroxine therapy. Subjects receiving thyroxine were identified from the computerized prescribing records of four general practices in the West Midlands. Of 18,944 patients registered, 146 (0.8%) were being prescribed thyroxine; 134 of these had primary hypothyroidism and the remainder had other thyroid or pituitary diseases prior to treatment. Of the 97 patients with primary hypothyroidism who agreed to have their thyroid stimulating hormone level measured, abnormal serum levels were found in 48%, high levels in 27% and low levels in 21%. There was a significant relationship between prescribed thyroxine dose and median serum thyroid stimulating hormone level: high hormone levels were found in 47% of those prescribed less than 100 micrograms thyroxine per day, while low levels were found in 24% of those prescribed 100 micrograms or more. Thus, thyroxine prescription was common in the four practices sampled, although indications for its use were appropriate.(ABSTRACT TRUNCATED AT 250 WORDS)

Early onset of lithium-associated hypothyroidism

In general practice, psychiatrists are confronted with the difficulty of structuring a rational design for the early detection of hypothyroidism. To determine the period during which a patient receiving lithium is most at risk of developing hypothyroidism, a retrospective study was conducted on the records of 154 patients at two general hospital lithium clinics from January 1980 to August 1991. Forty-two cases of hypothyroidism (clinical hypothyroidism and/or abnormally elevated levels of TSH) were detected. A significant difference was found between the onset of hypothyroidism and age (older patients developed more thyroid dysfunction), but no significant differences were found between thyroid abnormality and sex or diagnostic category and menopausal status, although trends were observed for the two former variables. This longitudinal study is the first to describe an outline of thyroid functioning in terms of the duration of treatment. Lithium-associated hypothyroidism develops most often during the first two years. Of the 42 cases of hypothyroidism, 16 were diagnosed within six months (38%), 23 within the first year (55%), and 31 two years (74%). Since thyroid functioning is an important parameter in the course of affective disorders, its close and frequent monitoring is mandatory during the first two years of treatment.

Equivalent discrimination among states of thyroid function by immunochemiluminimetric and immunoradiometric determination of thyrotropin

New immunochemiluminometric assays (ICMA) of thyrotropin (TSH) have been reported to facilitate the diagnosis of hyperthyroidism. To compare the accuracy of ICMA-TSH with that of a conventional immunoradiometric assay (IRMA-TSH), we examined 115 consecutive patients of a thyroid outpatient clinic. On the basis of complete thyroid-function testing, including thyroliberin tests, the untreated patients (n = 89) were subclinically hyperthyroid (n = 21), subclinically hypothyroid (n = 13), or euthyroid (n = 55). The receiver-operating characteristic (ROC) curve, used for comparing the TSH values obtained by these two methods, was shifted to the left for the IRMA-TSH. The area under the curve was greater for IRMA-TSH than for ICMA-TSH (0.984 vs 0.869, respectively), which suggests equal or better clinical performance of IRMA-TSH vs ICMA-TSH in discriminating between hyperthyroidism and euthyroidism. Both assays displayed similar clinical sensitivity/specificity for evaluating thyroid function in hypothyroid and treated patients. We conclude that the ICMA-TSH tested is not more accurate than an IRMA in distinguishing between hyperthyroidism and euthyroidism.

[Leipzig neonatal screening center--experiences with introduction of screening for congenital hypothyroidism]

The development of neonatal screening for congenital hypothyroidism in the GDR and particularly in Saxony after German reunification is described in this paper. The results of the studies and the experiences in respect of realisation and organisation of the screening for hypothyroidism in the screening centre of Leipzig are discussed. Measurement of TSH in the blood spot of about 65,000 newborn yielded an incidence rate of congenital hypothyroidism of 1:3,200. The onset of therapy within the first three weeks of life has been ensured until now since no children with congenital hypothyroidism were involved in cases of occasionally delayed blood sampling and/or postal delay. However, there has been some organisational disparity due to individually different handling of health insurance fund payments of screening fees for hypothyroidism in Saxony and also in the other federal states. The disadvantages of this lack of uniform regulations are explained. Neonatal screening for evidence of congenital adrenal hyperplasia by determining 17-alpha-hydroxyprogesterone, is under preparation.

Altered muscle sympathetic nerve activity in hyperthyroidism and hypothyroidism

To determine whether sympathetic nerve activity is altered in hyperthyroidism and hypothyroidism we microneurographically measured muscle sympathetic nerve activity in patients with these thyroid dysfunctions and compared the results with those of normal controls. Patients with hyperthyroidism tended to have less muscle sympathetic nerve activity than normal controls, and patients with hypothyroidism had significantly greater muscle sympathetic nerve activity than normal controls (P < 0.05). In all subjects, there was a significantly negative-correlation between the serum concentration of free triiodothyronine or free thyroxine and muscle sympathetic nerve activity, and there was a significantly positive correlation between the serum concentration of thyroid-stimulating hormone and the muscle sympathetic nerve activity. These results suggest an inverse relationship between thyroid function and sympathetic nerve activity.

Thyrotropin (TSH)-releasing hormone stimulation test responses employing third and fourth generation TSH assays

TRH stimulation tests (n = 1109) were performed on 1061 ambulatory and 43 hospitalized patients with varying thyroid status, using a TSH immunochemiluminometric assay with third and fourth generation sensitivity characteristics (functional sensitivity, 0.01 and 0.001 mU/L, respectively). TRH test results were analyzed as both absolute (stimulated minus basal TSH) and fold (stimulated/basal TSH) responses. The absolute TRH response varied 8-fold across the physiological TSH range, whereas the mean fold response remained almost constant (mean +/- SEM, 8.5 +/- 0.2). The fold response became progressively attenuated as basal TSH values declined below physiological levels, becoming essentially absent in clinically thyrotoxic patients with markedly depressed basal serum TSH levels (0.007 +/- 0.002 mU/L). Progressive attenuation also occurred at hypothyroid TSH levels; a markedly impaired fold response (2.5 +/- 0.4) was characteristic of primary hypothyroid patients with basal TSH values greater than 50 mU/L. In untreated central hypothyroid patients with near-normal basal TSH levels, the TRH fold response was impaired (1.7 +/- 0.2), whereas in T4-replaced central hypothyroid patients, fold responses were near normal (5.6 +/- 1.2). Neither nonthyroidal illness, age, or sex appeared to influence the pattern of fold TRH response in the populations evaluated. When using third and fourth generation TSH methodology, the TRH-stimulated TSH fold response is more diagnostically useful than the absolute TRH response. However, if patients have an intact hypothalamic-pituitary axis, there appears to be no diagnostic advantage gained by TRH testing over an accurately measured basal TSH value.

Autoimmune hepatitis and hypothyroidism associated with anti-thyroid hormone autoantibodies

A 40-year-old hypothyroid female who had been treated with synthetic thyroxine was admitted to our hospital in October 1988 due to abnormal liver function tests. She had low serum free triiodothyronine (T3; 2.3 pg/ml) and high serum thyrotropin (TSH; 20.8 microU/ml) concentrations. On the other hand, the serum free thyroxine (FT4) level was inappropriately high, being 2.46 ng/dl. Immune precipitation of radiolabeled thyroid hormones with her serum disclosed the binding of 125I-T3 and 125I-T4 to the extent of 9.5% and 11.3%, respectively (normal ranges for 125I-T3 and 125I-T4 binding are less than 6.3% and 5.9%, respectively). 125I-T4 binding to the patient's serum gamma globulin was completely displaced with the addition of unlabeled T4. Further examination disclosed that anti-T4 antibodies in her serum belong to IgG kappa class immunoglobulin.

Spectrum of serum thyroglobulin elevation in congenital thyroid disorders

Serum thyroglobulin (Tg) data are presented for 47 infants with congenital thyroid disorders. Abnormal elevation of serum Tg (> 250 micrograms/L) occurred in 17% of the population studied, whereas values in excess of 1,000 micrograms/L were demonstrated in 11% of infants. The latter group includes the first report of supraphysiologic Tg elevation in an infant with thyroid gland ectopia, and the highest reported thyroglobulin level in the syndrome of generalized thyroid hormone resistance in an infant homozygous for a novel deletion in the c-erbA beta receptor. Mechanisms involved in the pathogenesis of Tg elevation are discussed. We conclude that Tg elevation in congenital thyroid disorders is more common than previously recognized, and values > 1,000 micrograms/L identify infants with a spectrum of anatomic and biochemical abnormalities.

Subclinical hypothyroidism: a determinant of polycystic ovary syndrome

The present study was an endeavor to explore whether and how hypothyroidism plays a role in the etiology of polycystic ovarian syndrome (PCOS). A composite picture of the hormone profile was assessed in different groups of subjects (control women and hypothyroid women with or without PCOS). Comparative analysis of the results suggests that hypothyroidism is invariably followed by a lowering of sex hormone binding globulin and an increment in the free testosterone level, but further metabolism of testosterone (T) may or may not be directed towards an overproduction of estriol (E3). The factors that dictate the route of T metabolism, and the way by which E3 acts to rescue the ovaries from the development of PCOS under the hypothyroid state are discussed.

A radioimmunoassay for measurement of thyroxine sulfate

A highly sensitive, specific, and reproducible RIA has been developed to measure T4 sulfate (T4S) in ethanol extracts of serum. rT3 sulfate (rT3S) cross-reacted 7.1%, and T3S cross-reacted 0.59% in the RIA; T4, T3, rT3, and 3,3'-diiodothyronine cross-reacted 0.004% or less. The recovery of nonradioactive T4S added to serum averaged 95%. The detection threshold of the RIA was 18 pmol/L. The coefficient of variation averaged 6.9% within an assay and 12% between assays. T4S was bound by T4-binding globulin and albumin in serum. The free fraction of T4S in four normal sera averaged 0.06% compared to a value of 0.03% for T4 (P < 0.001). The serum concentration of T4S was (mean +/- SE) 19 +/- 1.2 pmol/L in normal subjects, 33 +/- 10 in hyperthyroid patients with Graves' disease, 42 +/- 15 in hypothyroid patients, 34 +/- 6.9 in patients with systemic nonthyroidal illnesses, 21 +/- 4.3 in pregnant women at 15-40 weeks gestation, and 245 +/- 26 in cord blood sera of newborns; the value in the newborn was significantly different from normal (P < 0.001). The mean concentration of T4S in amniotic fluid samples at 15-38 weeks gestation was 106 +/- 22 pmol/L (cf. normal adults; P < 0.001). Administration of sodium ipodate (Oragrafin; 3 g, orally) to hyperthyroid patients was associated with a transient increase in serum T4S. The T4S content of the thyroid gland was less than 1/4000th that of T4. We conclude that 1) T4S is a normal component of human serum, and its levels are markedly increased in newborn serum and amniotic fluid; and 2) the sulfation pathway plays an important role in the metabolism of T4 in man.

Iodine-induced hypothyroidism in patients on regular dialysis treatment

Hypothyroidism with a serum concentration of thyroid-stimulating hormone (TSH) above 40 microU/ml was noted in 3 (3.2%) of 93 patients on regular hemodialysis or continuous ambulatory peritoneal dialysis. These 3 patients had no history of thyroid disease and were receiving no medication known to influence thyroid function. They had habitually eaten iodine-rich foods and showed an enlarged thyroid gland with a preserved radioactive iodine uptake and a markedly elevated serum inorganic iodine (II) level. In all 3 patients, both thyroidal microsomal antibody and thyroglobulin antibody titers measured by hemagglutination methods were less than 100, and TSH-binding inhibitory immunoglobulin was negative. Moreover, histologically no lymphocytic infiltrations were observed. With only iodine restriction, serum TSH level markedly decreased from 44.6 to 3.6 microU/ml in case 1, from 90.6 to 3.2 microU/ml in case 2 and from 43.2 to 9.4 microU/ml in case 3 in parallel with decreases in the serum II level. These results suggest that at least in an area like Japan, where the daily intake of iodine is high, iodine-induced hypothyroidism may be induced in patients undergoing regular dialysis treatment even in the absence of apparent underlying thyroid disease.

Effect of environmental and hypothalamic factors on thyrotropin secretion in the hypothyroid rat

1. Fourteen days after hypothyroidism was induced either by propylthiouracil (PTU) treatment or by thyroidectomy, the serum thyrotropin (TSH) responses to morphine (5 or 20 mg/kg bw), ether stress (30 min) and cold exposure (60 min) were compared with those in normal rats. 2. The decrease in serum TSH levels after morphine and ether stress found in the normal rats were abolished or much reduced respectively. 3. The increase in serum TSH in response to cold exposure and the diurnal rhythm of serum TSH (lower level at night) were also absent in the hypothyroid rat. 4. The stimulating effects of low dose of thyrotropin releasing hormone (TRH) and the inhibitory effects of somatostatin and apomorphine were completely abolished, while the stimulating effects of a high dose of TRH were much reduced in the hypothyroid rat. 5. These results indicate that in the hypothyroid rat the effect of a lack of negative feedback action of thyroid hormone predominates, and that hypothalamic factors are probably unimportant in the regulation of TSH secretion.

Urinary cortisol metabolites in the assessment of peripheral thyroid hormone action: application for diagnosis of resistance to thyroid hormone

Urinary cortisol metabolites are altered both quantitatively and qualitatively in thyroid dysfunction. This study was conducted to elucidate the usefulness of urinary cortisol metabolites in the assessment of peripheral thyroid hormone action, particularly in the patients with inappropriate thyrotropin secretion. Twenty-four hour urinary 17-hydroxycorticosteroid (17-OHCS) level and gas chromatographical steroid profile were studied in 25 hyperthyroid, 18 hypothyroid, and 24 euthyroid control subjects. Five patients with generalized thyroid hormone resistance and two patients with thyrotropin secreting pituitary tumor were also studied. The ratio of urinary tetrahydrocortisone to tetrahydrocortisol (THE/THF) was significantly elevated in hyperthyroidism (4.58 +/- 1.49) and depressed in hypothyroidism (1.31 +/- 0.55) compared to control (1.93 +/- 0.35). There were good correlations between THE/THF and serum thyroid hormone levels, especially in hypothyroidism. THE/THF can be a good biochemical indicator for deficiency of peripheral thyroid hormone action. Two patients with thyrotropin-secreting tumor showed high THE/THF, which reflected thyroid hormone excess. In contrast, THE/THF in the patients with generalized thyroid hormone resistance was low as compared to high serum thyroid hormone levels. Similar findings were demonstrated with 17-OHCS but discrimination of thyroid hormone resistance was insufficient. Thus, the ratio of the urinary concentrations of cortisol metabolites, THE/THF, appears to be a good marker for peripheral thyroid hormone resistance.

Basophilic leukocytes in hypothyroidism

Since in the literature basophilia is frequently related to myxedema, we evaluated basophilic leukocytes in patients with hypothyroidism, applying routine techniques used in clinical laboratories. The study included normal persons, untreated patients with hypothyroidism, and euthyroid subjects with hyperlipidemia. The number of circulating basophils was determined by differential counts of Pappenheim stained blood smears. No difference in relative and total basophil counts was detected in patients with hypothyroidism as compared to healthy controls (1.0% and 58.1 basophils/microliters vs. 0.8% and 50.8 basophils/microliters, respectively). The percentage of basophils in myxedema associated with hypercholesterolemia amounted to 1.0%, their absolute number to 57.6/microliters; in hypothyroid patients presenting normal serum cholesterol levels, the relative and absolute numbers of basophilic leukocytes was not statistically different (0.83% and 61.1 basophils/microliters, respectively). We conclude that in patients with hypothyroidism the number of basophils is not statistically different from the values of basophils in healthy controls. Furthermore, the number of peripheral blood basophils in hypothyroidism is not related to the serum cholesterol level.

Clinical characteristics of congenital hypothyroidism detected by neonatal screening

Various clinical features have been prospectively evaluated in 18 infants with congenital hypothyroidism detected by the National Taiwan University Hospital screening program from January 1990 to May 1991. During the same period of time, 24 suspected cases with normal thyroid function at referral were similarly evaluated and used as controls. Among the various clinical features evaluated, there were statistically significant differences between the two groups for the following five items: feeding problems, constipation, dry skin, umbilical hernia and enlarged posterior fontanel. However, these findings were subtle and nonspecific. Even when the neonatal hypothyroid index (Quebec) was used for evaluation, only one-half of these hypothyroid babies had scores of more than two. On the other hand, there was a statistically significant difference in the thyroid function test between the two groups whether it was determined from the filter paper spot or the serum. Our observations clearly demonstrate the difficulty in establishing a diagnosis of congenital hypothyroidism on clinical grounds only at an early age of life. This study also emphasizes the importance of biochemical screening in the early detection and treatment of hypothyroid infants.

Lipoprotein(a) and apolipoprotein B plasma concentrations in hypothyroid, euthyroid, and hyperthyroid subjects

Overt hypothyroidism is associated with premature coronary artery disease, and this is assumed to be due to a deteriorated metabolism of atherogenic lipoproteins. The effect of thyroid status on plasma concentrations of lipoprotein(a) [Lp(a)], a recently recognized highly atherogenic lipoprotein in man, is unknown. In a cross-sectional study, plasma Lp(a) concentrations were higher in overtly hypothyroid subjects [255 +/- 28 (+/- SD) mg/L; n = 19] and lower in hyperthyroid subjects (75 +/- 28 mg/L; n = 27) compared to those in 54 euthyroid subjects (150 +/- 36 mg/L) and a reference population of local blood bank donors (155 +/- 31 mg/L; n = 114). These findings were confirmed in a follow-up study of 19 hypothyroid and 8 hyperthyroid individuals. In the hypothyroid subjects, initial levo-T4 substitution therapy (25 micrograms daily) caused a 55% decrease in plasma Lp(a) concentrations and a 27% decrease in total plasma apolipoprotein B (apo B). Good agreement was found between the decrease in Lp(a) and apo B at a normal free T4 index. Follow-up of 8 hyperthyroid subjects revealed that their plasma Lp(a) and apo B concentrations significantly increased with return of euthyroidism. In conclusion, good agreement was found between the direction and magnitude of the responses of apo B and Lp(a) to changes in thyroid status. The following findings suggest that different thyroid hormone-dependent mechanisms modulate plasma Lp(a) concentrations in man, in part analogous to modulation of apo B: 1) impaired catabolism in the hypothyroid state, and 2) a combination of suppressed secretion of apoB and Lp(a) with increased catabolism in hyperthyroid subjects. Increased plasma Lp(a) concentrations may contribute to the increased risk of premature coronary artery disease in the hypothyroid state.

Early identification of congenital hypothyroid infants with abnormalities in pituitary setpoint for T4-induced TSH release

It is now clear that early detection and adequate replacement therapy of congenital hypothyroidism (CH) results in normal growth and psychomotor development. However, there is evidence that some of those infants might have a persistent alteration in the T4 feedback control of TSH release. To characterize further this phenomenon, 25 treated CH children were divided into two groups: group A consisted of children whose TSH was suppressed as early as 1 month after the onset of therapy, and group B consisted of children whose TSH suppression occurred much later. There were no differences in the etiology of CH, in the mean T4 and T3 serum levels or in the mean LT4 treatment dosage between the two groups. All children were clinically euthyroid throughout the follow-up, developed according to expected norms and no deviations were noted in bone age. However, serum TSH levels remained elevated in group B infants throughout the follow-up period (up to 14 years). Increase of LT4 treatment dosage resulted in TSH suppression in both groups. However, the TSH levels obtained in group B were still higher compared to group A. These results suggest that some CH infants might have an abnormal setpoint for T4 control of TSH secretion and that these infants can be detected as early as 1 month after birth. Thus, serum T4, T3 levels and clinical progress are better guides to the adequacy of therapy than serum TSH concentrations in this group of CH infants.

Hypothyroidism and anti-endothelial cell antibodies

Because of our previous demonstration of anti-endothelial cell antibodies (AECA) in patients with insulin-dependent diabetes mellitus and their association, in this condition, with thyroid disease, we sought these antibodies in patients with suspected thyroid dysfunction using an enzyme immunoassay with human umbilical vein endothelial cells as the substrate. AECA were found in 5/120 (4.2%) patients with normal and 15/97 (15.4%) with abnormal thyroid function. The increased prevalence in the latter group was due to a highly significant association between the presence of AECA and raised levels of TSH. We conclude that a highly significant correlation exists between the levels of AECA and TSH, but not between those of AECA and fT4. Patients with hypothyroidism as defined by high levels of TSH have AECA significantly more often than patients with low or normal TSH (22.2% versus 2.8% and 5.8%).

Endemic juvenile hypothyroidism in a severe endemic goitre area of Sudan

OBJECTIVE

The aim of the study was to assess thyroid function, iodine intake and exposure to dietary goitrogens of children living in an area with a high prevalence of goitre, in the region of Darfur, Sudan.

DESIGN

In a village where goitre affected approximately 85% of children, a cross-sectional survey of thyroid function was performed in children 0-7 years old.

PATIENTS

Twenty neonates and 190 children, aged 1 month to 7 years, were included.

MEASUREMENTS

Thyroid hormones, urinary iodide and thiocyanate excretion were measured.

RESULTS

Mean +/- SD serum T4 was below the normal range at birth (82 +/- 50 nmol/l) and in the age group less than 2 years (73 +/- 46). Children older than 2 years had even lower serum T4: 37 +/- 37 (P < 0.001) at 3-4 years and 36 +/- 38 (P < 0.001) at 5-7 years. Mean serum TSH was 25.8(6.2-107.7) mU/l at birth, 8.3(2.5-27.8) in the group less than 2 years, 15.3(2.9-79.1) at 3-4 years and 16.4(2.7-98.3) at 5-7 years. The overall prevalence of hypothyroidism (TSH > 50 mU/l) was 24%. Mean urinary thiocyanate was high at birth (107 +/- 69 mumol/l), normal in the group less than 2 years and higher in children older than 2 years (126 +/- 69 mumol/l) (P < 0.001). All age groups had a low urinary iodide concentration.

CONCLUSION

Hypothyroidism was very frequent in each age group. The higher frequency of hypothyroidism observed in weaned children (> 2 years) was attributed to the combined effects of iodine deficiency and goitrogens (thiocyanate and glycosylflavones) derived from millet.

Thyroid hormone levels affected by time of blood sampling in thyroxine-treated patients

Patients receiving exogenous levothyroxine are reported to have higher total and free serum thyroxine levels than euthyroid controls. This may be an artifact of the serum collection time. We explored the effect of collection time on serum levels of thyroid hormones in outpatients receiving levothyroxine for replacement therapy (26 patients) or suppression of thyrotropin (25 patients). Blood samples, obtained during regular clinic visits (random samples) and at more than 22 h from ingestion of levothyroxine (trough samples), were assayed for total and free thyroxine, triiodothyronine, and thyrotropin. Four athyreotic patients on levothyroxine therapy had serial blood sampling over 24 h. Compared to corresponding trough samples, random samples had elevated total thyroxine levels in patients receiving replacement (8.1 +/- 1.2%, mean +/- SE, p = 0.0001) and in patients undergoing suppression (8.8 +/- 1.6%, p = 0.0001). Free thyroxine was increased by 12.7 +/- 2.6% (p = 0.0003) and 14.5 +/- 2.3% (p = 0.0001), respectively, compared with trough samples. Thyrotropin levels were 18.9 +/- 6.8% (p = 0.003) lower in patients receiving replacement and triiodothyronine levels showed small or no changes. Time-course analysis showed that free and total thyroxine levels remained significantly elevated above baseline for 9 and 5 h, respectively, after a levothyroxine dose. In conclusion, there is a transient increase in thyroid hormone levels for 9 h after an oral levothyroxine dose. Accurate assessment of thyroid hormone levels in patients receiving levothyroxine therapy should take this into account. This has greatest significance in selecting minimal levothyroxine dosages for suppression of thyrotropin.

[Isotopic composition of blood plasma carbon in patients with endocrine diseases]

Forty-eight patients suffering from diabetes mellitus, obesity, hyper- and hypothyrosis, Icenko-Cushing's disease were examined to elucidate the relationship between blood plasma carbon isotopic composition and the type of the endocrine disease. This value varied within the range of -19.7 to -24.7/1000, the mean value being 5/1000. Blood plasma carbon in the diabetics is enriched for 12C light isotope (delta 13C from -23.0 to -24.5/1000) as against a similar characteristic in obese patients (delta 13C from -20.5 to 21.99/1000). Patients with hypo- and hyperthyrosis and the Icenko-Cushing's disease have a wider range of delta 13C values that seems to be explained by the Icenko-Cushing's disease heterogeneity and the presence of biorhythms. Clear-cut isotopic differences in the blood sera of adults and children were revealed whatever the disease type, these differences indicating the changes in cellular metabolism energy in the ontogenesis. These data can be satisfactorily explained within the frames of the model of cellular division of carbon isotopes, suggested previously.

Morbidity in patients on L-thyroxine: a comparison of those with a normal TSH to those with a suppressed TSH

OBJECTIVE

Patients on L-thyroxine with a 'suppressed' TSH (< 0.05 mU/l) were compared to those in whom TSH was detectable but not elevated (0.05-4.0 mU/l), with regard to morbidity data.

DESIGN

Biochemical data from Tayside Thyroid Register was matched to hospital admissions data obtained from Health Board Statistics.

PATIENTS

The patients were identified from those registered on the computerized Tayside Register.

MEASUREMENTS

Serum T4 and TSH assays, clinical assessment scores, and admission records with regard to ischaemic heart disease, overall fractures, fractured neck of femur and breast carcinoma.

RESULTS

Over one year, 1180 patients on thyroxine replacement had clinical and biochemical assessment; 59% had a suppressed TSH and 38% 'normal' TSH. Patients with a suppressed TSH exhibited higher median serum thyroxine levels (146 nmol/l, range 77-252 vs 119 nmol/l, 58-224; P < 0.001). Patients under the age of 65 years on L-thyroxine had an increased risk of ischaemic heart disease compared to the general population (female 2.7 vs 0.7%, P < 0.001; male 6.4 vs 1.7%, P < 0.01), but the risk was no different between those with suppressed and normal TSH. There was no increase in risk for overall fracture, fractured neck of femur or breast carcinoma in those on thyroxine with suppressed or normal TSH.

CONCLUSION

Patients under the age of 65 years on L-thyroxine had an increased risk of ischaemic heart disease. There was no excess of fractures in patients on L-thyroxine even if the TSH is suppressed.