Blood levels of 3,5,3'-triiodothyronine and thyroxine: differences between children, adults, and elderly subjects

Thyroid hormones and changes in body weight and metabolic parameters in response to weight loss diets: the POUNDS LOST trial

BACKGROUND

The role of thyroid hormones in diet-induced weight loss and subsequent weight regain is largely unknown.

OBJECTIVES

To examine the associations between thyroid hormones and changes in body weight and resting metabolic rate (RMR) in a diet-induced weight loss setting.

SUBJECTS/METHODS

Data analysis was conducted among 569 overweight and obese participants aged 30-70 years with normal thyroid function participating in the 2-year Prevention of Obesity Using Novel Dietary Strategies (POUNDS) LOST randomized clinical trial. Changes in body weight and RMR were assessed during the 2-year intervention. Thyroid hormones (free triiodothyronine (T3), free thyroxine (T4), total T3, total T4 and thyroid-stimulating hormone (TSH)), anthropometric measurements and biochemical parameters were assessed at baseline, 6 months and 24 months.

RESULTS

Participants lost an average of 6.6 kg of body weight during the first 6 months and subsequently regained an average of 2.7 kg of body weight over the remaining period from 6 to 24 months. Baseline free T3 and total T3 were positively associated, whereas free T4 was inversely associated, with baseline body weight, body mass index and RMR. Total T4 and TSH were not associated with these parameters. Higher baseline free T3 and free T4 levels were significantly associated with a greater weight loss during the first 6 months (P<0.05) after multivariate adjustments including dietary intervention groups and baseline body weight. Comparing extreme tertiles, the multivariate-adjusted weight loss±s.e. was -3.87±0.9 vs -5.39±0.9 kg for free T3 (P_trend=0.02) and -4.09±0.9 vs -5.88±0.9 kg for free T4 (P_trend=0.004). The thyroid hormones did not predict weight regain in 6-24 months. A similar pattern of associations was also observed between baseline thyroid hormones and changes in RMR. In addition, changes in free T3 and total T3 levels were positively associated with changes in body weight, RMR, body fat mass, blood pressure, glucose, insulin, triglycerides and leptin at 6 months and 24 months (all P<0.05).

CONCLUSIONS

In this diet-induced weight loss setting, higher baseline free T3 and free T4 predicted more weight loss, but not weight regain among overweight and obese adults with normal thyroid function. These findings reveal a novel role of thyroid hormones in body weight regulation and may help identify individuals more responsive to weight loss diets.

Thyroid and aging or the aging thyroid? An evidence-based analysis of the literature

Thyroid hormone production, metabolism, and action change with aging. The reference ranges for serum thyrotropin and thyroid hormones are derived mainly from younger populations. Thus, the prevalence of subclinical thyroid dysfunction is increased greatly in the elderly. However, it is unclear whether mild thyroid dysfunction in the elderly is associated with adverse outcomes. In this review, we discuss current evidence-based literature on thyroid function in the elderly and whether subclinical thyroid dysfunction in the elderly should be treated.

Relationship between urinary triclosan and paraben concentrations and serum thyroid measures in NHANES 2007-2008

Triclosan and parabens are broad spectrum antimicrobials used in a range of consumer products. In vitro and animal studies have suggested the potential for these compounds to disrupt thyroid function, though studies in humans have been limited. The objective of the study was to assess the relationship of urinary concentrations of triclosan and parabens with serum thyroid measures in a large, representative sample of the US population. We conducted an exploratory, cross-sectional analysis of data on urinary biomarkers of triclosan and paraben exposure and serum thyroid measures obtained from 1831 subjects (ages≥12 years) as part of the 2007-2008 National Health and Nutrition Examination Survey (NHANES). We found evidence of some inverse associations between parabens and circulating thyroid hormone levels in adults, with the strongest and most consistent associations among females. We also observed a positive association between triclosan and total triiodothyonine (T3) concentrations in adolescents. These results, in accordance with the in vitro and animal literature, suggest that paraben, and potentially triclosan, exposures may be associated with altered thyroid hormone levels in humans. Further research is needed for confirmation and to determine the potential clinical and public health significance of these findings.

Thyroid function tests in elderly Finnish men

Serum total thyroxine (T4), triiodothyronine (T3) and thyrotrophin (TSH) levels were determined among 65-84-year-old Finnish men living either in eastern Finland (n = 309) or in southwestern Finland (n = 389). The mean value for serum total T4 was 112.2 +/- 23.0 nmol/l in eastern Finland and 111.3 +/- 21.9 nmol/l in southwestern Finland. The mean value for serum T3 was 1.76 +/- 0.30 nmol/l in eastern Finland and 1.75 +/- 0.46 nmol/l in southwestern Finland. Serum TSH values showed the mean for men from eastern Finland to be 3.25 +/- 2.29 mU/l and the mean for men from southwestern Finland to be 3.11 +/- 1.83 mU/l. No differences were found in the means of the thyroid function tests between the two areas. Serum T4 levels were not related to age. Serum T3 values fell with age. In both areas, serum TSH levels were highest among men 70-74 years of age.

Thyroid function in a population of children with attention deficit hyperactivity disorder

OBJECTIVE

To determine the prevalence of thyroid hormone abnormalities and generalized resistance to thyroid hormone in a population of children with attention deficit hyperactivity disorder (ADHD) as compared to reference ranges determined from a control population and hence to determine if routine thyroid hormone screening in children with non-familial ADHD is indicated.

METHOD

Children attending the State Child Development Centre in Perth, Western Australia with ADHD, as defined by the Diagnostic and Statistical Manual of Mental Disorders (fourth edition) provided the study population. The control population consisted of 353 normal children with a history of allergy in whom radioallergosorbent (RAST) testing was being performed.

RESULTS

The prevalence of thyroid hormone abnormalities in the study population was 2.3% (95% CI 0.6%, 5.7%). There were no cases of generalized resistance to thyroid hormone. The prevalence of thyroid hormone abnormalities in the general population of children and adolescents has been reported to vary between 1 and 3.7%.

CONCLUSION

Routine thyroid hormone screening is not indicated in children with non-familial ADHD.

Skeletal responsiveness to thyroid hormone is not altered at menopause

Hyperthyroidism is characterized by increased bone turnover and resorptive activity. Similar changes in remodeling are seen after menopause. To study the role of thyroid hormone in the menopause-related changes in bone metabolism, we investigated thyroid status and the sensitivity of bone to thyroid hormone in 14 premenopausal and 15 early postmenopausal women. Triiodothyronine (T3) was administered to the two groups as 20 micrograms doses three times daily for 7 days. The skeletal response was assessed by monitoring bone alkaline phosphatase (BAP), osteocalcin (BGP), pyridinium crosslinked telopeptide domain of type I collagen (ICTP) in serum and urinary excretion of hydroxyproline (OHP), pyridinoline (PYR), and deoxypyridinoline (DPR) at days 0, 8, 15, and 57. The early postmenopausal women had increased bone turnover as reflected in sBAP (p < 0.05), sBGP (p < 0.05), and uOHP (p < 0.01) when compared with premenopausal controls. T3 stimulation of early postmenopausal and premenopausal women significantly increased the markers of bone resorption: sICTP (56% vs. 44%), uOHP (45% in both groups), and UPYR (83% vs. 17%) without any significant differences between groups. Of the formative markers, only sBGP increased significantly after stimulation (34% vs. 41%), but both sBGP and sBAP displayed significant increases from days 15 to 57. Thus, stimulation with thyroid hormone results in an immediate stimulation of ongoing bone formation and bone resorption, but also initiation of new remodeling which, after 8 weeks, reached the formative phase. PTH decreased (p < 0.01) in both groups but serum calcium and serum phosphate were unaltered. In conclusion, menopause is not characterized by altered levels of thyroid hormones or altered skeletal responsiveness to thyroid hormones.

Thyroid hormones homeostasis in pediatric patients during and after cardiopulmonary bypass

The concentrations of thyroid hormones were measured in 14 pediatric patients before, during, and after cardiopulmonary bypass. The ages of the patients ranged between 18 months and 14 years. Patients were kept normothermic, or moderate or deep hypothermia was induced depending on the specific pathologic condition involved. A marked reduction in the levels of total triiodothyronine, total thyroxine, free triiodothyronine, and thyroid-stimulating hormone, and in the ratio of free triiodothyronine to free thyroxine was detected during the time frame of the study. The minimum levels of each hormone were reached between 12 and 48 hours after cardiopulmonary bypass, indicating that changes in thyroid function and in the conversion of thyroxine to triiodothyronine are triggered by cardiopulmonary bypass and represent specific phenomena, and that these changes are progressively exacerbated during the post-operative period. The thyroid-stimulating hormone level was markedly reduced versus its baseline values (24% +/- 0.13%), despite low levels of both total (40% +/- 18%) and free (39% +/- 20%) triiodothyronine: it returned to its preoperative level by the third postoperative day, but both the total (75% +/- 10%) and free (74% +/- 3%) triiodothyronine levels remained below their baseline values for 7 days postoperatively. Neither hemodilution nor hypothermia was responsible for the alteration observed. We conclude that pediatric patients undergoing cardiopulmonary bypass manifest changes in hormone metabolism similar to those seen in adult patients. These changes increase progressively during the postoperative period, and are still present 7 days postoperatively. The exact mechanism responsible for causing these changes is not thoroughly understood. Whether triiodothyronine replacement therapy is beneficial or deleterious remains controversial.

Interpretation of thyroid function tests

The development of radioimmunoassays over the past 20 years has expanded our knowledge of thyroid physiology and improved our management of thyroid disease. The use of these tools in neonatal screening for congenital hypothyroidism alone has reduced the incidence of mental retardation in the industrialized world. Based on an accurate physical examination, the judicious use of immunoassays for thyroxine, triiodothyronine, and thyrotropin and the use of thyroglobulin and thyroid microsomal antibodies will allow the general physician to confidently delineate common thyroid disorders in the great majority of patients. The additional use of an ultrasensitive thyrotropin assay, thyroid scans, and fine-needle aspiration biopsy will complete the accurate diagnosis of the great majority of thyroid diseases. The ultrasensitive thyrotropin assay may become the universal thyroid function test. The major pitfalls in the use of these tests lies in the variable effect chronic illness has on the most frequently used tests: thyroxine and triiodothyronine. Tests for these thyroid hormones, which in the relatively well outpatient are highly accurate, may in the ill, hospitalized patient become very misleading.

Thyroid hormone indices in adult healthy subjects: no influence of aging

Controversy exists regarding the influence of aging on thyroid hormone metabolism. Several investigators report lowering of T3 and/or a rise in reverse T3 (rT3) in elderly subjects. Others suggest that these thyroid hormone alterations were secondary to associated disorders rather than old age, and questioned the "healthy" status of the subjects studied in the earlier reports. Therefore, to assess the possible effect of aging we studied T3 resin uptake, T4, free T4, T3, and rT3 concentrations in 152 euthyroid healthy adult subjects. These subjects were selected carefully and were therefore devoid of any illness, acute or chronic, and were not treated with any medications at the time of study. No significant alterations were noted in any of the thyroid hormone concentrations in subjects divided into groups according to age. Nor was there a significant difference in these parameters between men and women of any individual age group or for all ages combined. Therefore, old age per se may not influence thyroid hormone metabolism and hence may not induce changes in serum thyroid hormone concentrations. The changes in thyroid hormones noted previously in elderly subjects may be a reflection of concurrent disorders and not old age.

Thermogenic effect of food in physically well-trained elderly men

Basal metabolic rate (BMR) and the thermogenic effect of food (TEF) after a liquid mixed meal of 2092 kJ (500 kcal) were examined in physically well-trained, elderly men in comparison with sedentary weight- and age-matched controls. BMR tended to be higher and TEF was significantly higher in the physically well-trained men than in the controls. No certain differences were found in plasma thyroid hormones or catecholamines. BMR correlated with whole body potassium while TEF did not. The tendency to elevated BMR in the well-trained men might therefore be due to their greater muscle mass. The elevated TEF, however, probably has other causes and might be associated with the elevated catecholamine sensitivity associated with the physically trained condition.

Low triiodothyronine and raised reverse triiodothyronine levels in patients over fifty years of age who have type II diabetes mellitus: influence of metabolic control, not age

Several studies have demonstrated that the uncontrolled diabetic state in both type I as well as type II diabetes mellitus is characterized by altered thyroid hormone metabolism, which results in the lowering of serum triiodothyronine (T3) levels and a reciprocal elevation of T3 (rT3) levels. Because the majority of type II diabetics are over 50 years of age and because numerous previous reports have implicated aging as a cause of low T3 and high rT3 levels, we studied 220 type II diabetics from 40-85 years of age to assess the influence of aging and metabolic control on thyroid hormone levels. Serum thyroxine (T4) free T4, T3 resin uptake, and thyroid-stimulating hormone (TSH) measurements in diabetic patients were not significantly altered compared with 37 young normal control subjects, irrespective of age or the grade of metabolic control. Serum T3 levels declined and rT3 levels rose in the diabetic patients with worsening of the metabolic control. However, with comparable metabolic control, the levels were not significantly different from the younger patients. Therefore, low T3 and high rT3 levels observed in patients of any age who have type II diabetes mellitus may be exclusively caused by deranged metabolic control of their disease.

Factors influencing serum free T4 in 70-year-old men. Implications for reference intervals in the elderly

We tested a procedure to obtain reference values based on careful clinical analysis of a population sample with the background (a) that there are conflicting reports of age effects on thyroid function parameters, possibly reflecting differences in numbers of sick and drug-treated subjects in the studied populations, and (b) that recommended procedures for the selection of reference subjects in elderly populations may give unacceptably high exclusion rates. Serum free T4 was determined by a ligand-analogue technique in a representative population of 460 males at age 70. The determinations were performed 5 years after the clinical study, and the results were analysed in relation to information obtained at the study as well as during a 5-year follow-up. The free T4 concentration was significantly low in subjects with neoplastic disease and subjects with chronic bronchitis and significantly high in subjects taking non-selective beta-adrenergic blockers. Selective beta 1-blockers had no apparent effect. Information on death during the 5 years after the study proved to have no significance. The free T4 concentration was found to be negatively correlated to body mass and body mass index, a correlation which has previously not been shown. Smoking was associated with a significantly low body mass but had no apparent effect on mean free T4 concentration. A procedure with successive exclusions of clinically defined diagnostic groups with significantly low or high analyte concentration and/or body mass may be used to obtain reference values for thyroid function and may reveal previously unknown relationships.

Thyroid function in the elderly

Thyroid dysfunction is common in older individuals, yet the diagnosis is often complicated by atypical clinical presentations and difficulty in interpretation of laboratory tests. An understanding of the alterations in thyroid function occurring normally as a consequence of the aging process is necessary for correct laboratory diagnosis of thyroid dysfunction in the elderly. There are subtle alterations in hypothalamic and pituitary function but normal feedback control of TSH secretion persists. In the thyroid itself, morphologic changes develop with age, but have little impact on thyroid hormone economy. Thyroidal secretion of thyroxine decreases, but parallels the decrease in thyroxine degradation rate, resulting in unaltered plasma thyroxine levels. Decreased peripheral conversion of thyroxine to triiodothyronine causes a fall in triiodothyronine concentrations. Nonthyroidal illnesses in the elderly may perturb the laboratory assessment of thyroid function by producing isolated high or low thyroxine levels in euthyroid individuals.

Thyroid function and thyroid hormone metabolism in elderly people. Low T3-syndrome in old age?

T4-, T3- and reverse-T3 concentrations were measured in the sera of 365 subjects beyond the age of 65 in order to evaluate if the decrease of serum T3 frequently observed in old age can be attributed to old age per se or to concomitant nonthyroidal disease. The results obtained from a carefully selected healthy group of elderly people show that 1) total and free T3 levels are lower in senescence but well within the range for euthyroidism in younger healty controls;2) the decrease of serum T3 is more pronounced and occurs earlier in healthy old males than in females, so that for subjects over the age of 75, the upper limit for euthyroidism has to be adjusted by 10% in women and by 20% in men; and 3) there is no low T3 syndrome characterized by decreased serum T3 and increased serum reverse T3, solely due to old age. Turnover kinetics have shown the daily production of T4 and T3 in old age to decrease by 20 micrograms and 10 micrograms, respectively, and an increased T3 metabolic clearance not to account for the reduction of serum T3 concentrations. Combined stimulation tests with TSH and TRH showed that the functional reserve of the thyroid gland to produce T3 is maintained in old age. The first step in the sequence of events may be seen in an impairment of TSH secretion leading to an adaptation of the amount of thyroid hormones to a reduced mass of metabolically active body tissue in old age.

Thyroid hormones and lipolysis in physically trained rats

In rats a single bout of exercise resulted in increased triiodothyronine (T3), thyroxine (T4), and triiodothyronine/reverse triiodothyronine (T3/rT3) ratio 20 hr after exercise. The effect of norepinephrine on lipolysis in vitro was potentiated. In trained rats no changes were found in T4, T3, or rT3 concentrations. The T3/rT3 ratio as well as basal and stimulated TSH concentrations decreased in comparison with sedentary, freely eating rats. Moderate food restriction to produce a body weight similar to that of trained animals caused no changes in T4, T3, or rT3 concentrations but caused a decrease in T3/rT3 and in TSH levels. Training and moderate food restriction groups were not different. T3 in vitro caused a potentiation of catecholamine induced lipolysis in trained and food-restricted animals. With aging the serum concentration of T3 decreased and that of rT3 increased. Acute and chronic exercise both exert an effect on peripheral hormonal responses of lipolysis, while they have different and opposite effects on thyroid hormone concentrations. Physical training seems to have effects in this regard similar to those of moderate energy intake restriction. The results suggest that changes in peripheral effects of thyroid hormones during training should attract more attention.

Thyroid status in elderly sick patients

Thyroid investigations were performed on 55 euthyroid patients hospitalized for chronic disease or recovering from acute illness. Three age groups were considered. Abnormalities were found only in patients over 75 years: 1) in the TRH test the maximum increase in serum TSH (delta TSHmax) was less marked and frequently delayed to 60 min, whereas the increase in T3 in 120 min (delta T3) was maintained; basal TSH was normal; 2) in 15 out of 28 cases, the pattern of circulating thyronines was abnormal showing an increase in reverse T3 a normal or decreased T3, and a rise in free T4 index beyond the normal range in 3 cases. In all cases but one, the combination of delta TSHmax, and delta T3 differentiated these patients from hyperthyroids.

Serum concentrations of the iodothyronines in elderly subjects: decreased triiodothyronine (T3) and free T3 index

In four groups of subjects free of thyroid disease, the following determinations were made: serum concentrations of thyroxine (T4), triiodothyronine (T3), reverse T3(rT3) and diiodothyronine (T2), and calculated indices of free thyroxine (FT4) and free triiodothyronine (FT3). Group A comprised healthy subjects aged 16-64; Group B, 24 healthy elderly subjects aged 68-95; Group C, 23 elderly patients with mild well-controlled chronic illnesses, aged 70-85; Group D, 40 nursing home residents aged 66-100. Serum T4 and T2 concentrations and the FT4 index were not affected by age; the rT3 concentration was slightly but significantly elevated only in Group D patients. Serum T3 concentration was significantly lower in all groups of elderly subjects and decreased FT3 index measurements were detected after age 75. It was concluded that old age, without complicating illness, is accompanied only by a decrease in the serum level of T3 and the FT3 index; values for other iodothyronines are unchanged. Clinicians should consider the age-related changes in T3 and FT3 values when interpreting thyroid function tests.

Value of the free triiodothyronine index in the diagnosis of hyperthyroidism

Normalized T4 (T4N), total T3, T3 resin sponge uptake (T3U) and the response of TSH to TRH were determined in 264 patients sent for diagnosis of thyroid function. Many of these patients were diagnostic problem cases. Those with disease or medication known to invalidate the TRH test, but not those with abnormal thyroid hormone binding capacity in serum, were excluded. A free T3 index (FT3 index) was calculated as the product of total T3 and T3U. The FT3 index tended to decline with increasing age, the mean value being significantly higher in the youngest patients (12-20-years-old) than in the older age groups. In the diagnosis of hyperthyroidism, the FT3 index was superior not only to T4N but also to total T3, particularly in the presence of elevated hormone binding capacity (relatively low T3U). It is anticipated that the FT3 index will be most useful as a routine test for hyperthyroidism, especially in several (though not all) instances where the TRH test is invalidated.

Free-thyroxine index in psychotic and neurotic depression

The mean free-thyroxine index value of a group of 25 psychotic depressive patients was found to be significantly lower than that of an age- and sex-matched group of 20 neurotic depressives (P less than 0.01). These findings support the view that psychotic and neurotic depression are differentiated not only with respect to their clinical expression, but also on account of their biological substrate. Comparison of free-thyroxine index values of each group of patients with a group of 240 euthyroid subjects hospitalized for various somatic illnesses revealed a lower mean free-thyroxine index value in the group of psychotic depressives (P less than 0.001). The neurotic depressives were not differentiated from the group of euthyroid subjects. The decreased thyroid activity in psychotic depression might be interpreted in the light of recent findings implicating catecholamines in both the release of hypothalamic hormones and the neurochemical mechanism of the affective disorders.