Variable thyrotropin response to thyrotropin-releasing hormone after small decreases in plasma thyroid hormone concentrations in patients of advanced age

Age-Related Resistance to Thyroid Hormone Action

The age-related resistance to thyroid hormones (THs) explains the paucity of symptoms and signs of hyperthyroidism in older adults and may partly explain the myriad of symptoms and signs of hypothyroidism in biochemically euthyroid older people. This review considers the available data on the mechanisms underlying TH resistance with aging and compares these physiologic changes with the changes observed in congenital TH resistance syndromes. Aging is associated with alterations in TH economy along with a host of changes in the responsiveness of various tissues to THs. The age-related resistance to THs can be attributed to decreased TH transport to tissues, decreased nuclear receptor occupancy, decreased activation of thyroxine to triiodothyronine, and alterations in TH responsive gene expression. Although an increase in serum TH levels is expected in syndromes of TH resistance, unchanged serum TH levels in the euthyroid elderly is the result of increased sensitivity to TH negative feedback with increased suppression of thyroid-stimulating hormone, decreased thyroidal sensitivity to thyroid-stimulating hormone, and decreased TH production and secretion. The current clinical evidence suggests that the age-related TH resistance is mostly an adaptive response of the aging organism. It is tempting to speculate that similar changes can occur prematurely in a group of younger people who present with signs and symptoms of hypothyroidism despite normal serum thyroid function tests.

Thyrotropin-releasing hormone-induced growth hormone secretion in dogs with primary hypothyroidism

Primary hypothyroidism in dogs is associated with increased release of growth hormone (GH). In search for an explanation we investigated the effect of intravenous administration of thyrotropin-releasing hormone (TRH, 10 microg/kg body weight) on GH release in 10 dogs with primary hypothyroidism and 6 healthy control dogs. The hypothyroid dogs had a medical history and physical changes compatible with hypothyroidism and were included in the study on the basis of the following criteria: plasma thyroxine concentration < 2 nmol/l and plasma thyrotropin (TSH) concentration > 1 microg/l. In addition, (99m)TcO(4)(-) uptake during thyroid scintigraphy was low or absent. TRH administration caused plasma TSH concentrations to rise significantly in the control dogs, but not in the hypothyroid dogs. In the dogs with primary hypothyroidism, the mean basal plasma GH concentration was relatively high (2.3+/-0.5 microg/l) and increased significantly (P=0.001) 10 and 20 min after injection of TRH (to 11.9+/-3.5 and 9.8+/-2.7 microg/l, respectively). In the control dogs, the mean basal plasma GH concentration was 1.3+/-0.1 microg/l and did not increase significantly after TRH administration. We conclude that, in contrast to healthy control dogs, primary hypothyroid dogs respond to TRH administration with a significant increase in the plasma GH concentration, possibly as a result of transdifferentiation of somatotropic pituitary cells to thyrosomatotropes.

Chronobiology in the endocrine system

Biological signaling occurs in a complex web with participation and interaction of the central nervous system, the autonomous nervous system, the endocrine glands, peripheral endocrine tissues including the intestinal tract and adipose tissue, and the immune system. All of these show an intricate time structure with rhythms and pulsatile variations in multiple frequencies. Circadian (about 24-hour) and circannual (about 1-year) rhythms are kept in step with the cyclic environmental surrounding by the timing and length of the daily light span. Rhythmicity of many endocrine variables is essential for their efficacy and, even in some instances, for the qualitative nature of their effects. Indeed, the continuous administration of certain hormones and their synthetic analogues may show substantially different effects than expected. In the design of drug-delivery systems and treatment schedules involving directly or indirectly the endocrine system, consideration of the human time organization is essential. A large amount of information on the endocrine time structure has accumulated, some of which is discussed in this review.

Age modifies the pituitary TSH response to thyroid failure

OBJECTIVE

To investigate the association between serum TSH, total T4 and various patient characteristics when hypothyroidism is diagnosed in a population, and to study how age, sex and serum T4 levels influenced pituitary TSH response.

DESIGN

A computer-based register linked to laboratory databases prospectively identified all patients with new, biochemically overt hypothyroidism (n = 685) in an open cohort in Denmark. The diagnosis was verified in each patient, and disease was classified into nosological type. Serum TSH and total T4 were recorded at the time of diagnosis in untreated patients with spontaneous autoimmune hypothyroidism (n = 578).

MAIN OUTCOME

In untreated primary, spontaneous autoimmune hypothyroidism, we observed a four fold difference in average serum TSH levels between the youngest (0-20 years: TSH = 100 mU/l) and the oldest (80+ years: TSH = 24.4 mU/l) group of patients. No age dependent variation was observed in serum total T4. Log TSH showed an inverse linear correlation with age. An inverse linear correlation was present between log TSH and total T4 in both young and old patients, but for all total T4 values we observed lower median serum TSH values in elderly patients.

CONCLUSIONS

For the same degree of thyroid failure, the serum TSH is lower among the elderly. This is most likely caused by a decrease in the hypothalamic/pituitary response to low serum T4. A certain increase in serum TSH may indicate more severe hypothyroidism in an old than in a young patient, and longer time may be needed after thyroid hormone withdrawal before elderly patients with thyroid cancer reach sufficiently high TSH values to allow for an effective radio-iodine treatment.

Clinical, biochemical and therapeutical aspects of amiodarone-induced hypothyroidism (AIH) in geriatric patients with cardiac arrhythmias

The clinical features and the laboratory aspects of the amiodarone-induced hypothyroidism (AIH) in the elderly as well as the effects of amiodarone treatment in aged AIH people have not yet been well clarified. In the present paper, we evaluated 18 subjects of both sexes (7 females, 11 males), aged 65-83 years, affected by AIH, recruited in Central Tuscany, Italy. The patients were divided in two subsets on the basis of thyroid stimulating hormone (TSH) values: mild (TSH < 20 mU/l; Group A, n=11) and severe (TSH > 20 mU/l; Group B, n=7) hypothyroid patients. On the basis of clinical features, hypothyroidism was diagnosed only in two patients (out of Group B). Concerning the hormonal pattern, we found that free tetraiodothyronine (fT4) levels were significantly lower than the normal range only in Group B subjects; TSH and thyroglobulin were higher than normal in both groups; free triiodothyronine (fT3) were always in the normal range. Thyroid autoantibodies were found positive only in one patient out of Group A and in two patients out of Group B. In 5/18 patients T4 substitutive therapy was rapidly assigned, because of severe degree of hypothyroidism. In the remaining 13/18 patients, we evaluated the clinical behavior of AIH. After additional cardiac evaluation, amiodarone was withdrawn in 5/13 patients: during follow-up period (4-10 months) four patients became quickly euthyroid while one worsened. In 8/13 patients, amiodarone treatment had to be carried on; during follow-up (2-48 months), four patients remained mildly hypothyroid, while other four patients became severely hypothyroid. In conclusion, in amiodarone treated elderly people, diagnosis of hypothyroidism is reliable only on the basis of high values of TSH; clinical features and fT3 serum levels never enable diagnosis.

Three-year follow-up of borderline congenital hypothyroidism

The purpose of this study was to determine whether children with borderline hypothyroidism in the neonatal period had persistent hypothyroidism after 3 years of levothyroxine replacement therapy. Fourteen term infants with slightly abnormal newborn screening results (thyroxine <10th percentile, thyroid stimulating hormone ¿TSH <40 microU/mL) were identified. The subsequent serum confirmatory TSH results of 12 subjects were modestly elevated (5.3 to 18.8 microU/mL, normal 0.6 to 4.6), whereas 2 subjects who had borderline confirmatory TSH (4.6 and 4.7 microU/mL) had abnormal TSH responses to thyrotropin releasing hormone testing. After 3 years of therapy, levothyroxine was discontinued in 13 patients, and repeat thyroid function tests were obtained 1 month later. Levothyroxine was not discontinued in one patient because he had an elevated random TSH (10 microU/mL) while receiving therapy. At 3 years of age, 13 patients had persistently abnormal thyroid function tests (TSH >4.6 microU/mL or a thyroid releasing hormone test result consistent with primary hypothyroidism), and levothyroxine was reinitiated. Only one patient had normal thyroid function studies. Although prospective studies are still lacking, we recommend levothyroxine replacement in newborns with borderline hypothyroidism.

Thyrotoxicosis in old age: a different clinical entity?

Thyrotoxicosis generally presents with classic signs and symptoms in younger people. Among the elderly population atypical presentation is recognized, although this has not been well quantified or characterized. To avoid misdiagnosis or delay in diagnosis, clinical suspicion needs to remain high.

Prevalence of thyroid hormone abnormalities in elderly patients with symptoms of organic brain disease

Analysis of the serum concentrations of free thyroid hormones (fT3, fT4) and thyrotropin (TSH) in 173 psychogeriatric patients (94 females and 79 males, mean age 79 +/- 8 years) disclosed that the hormone levels were related to sex, psychiatric diagnosis, medication and the presence of nonthyroid illness (NTI). Subnormal concentrations of thyroid hormones and/or TSH were found in 25% of the patients. In addition, fT3 and fT4 concentrations were significantly lower (p < 0.05 and p < 0.001, respectively) in demented males compared with demented females although the levels were within the reference limits. Strongly negative correlations between fT3 and age (p < 0.001), and between fT3 and the sedimentation rate (SR) (p < 0.01) were found in demented but not in non-demented patients. These correlations were most pronounced in (age) or restricted to (SR) demented males. In addition, the correlation between fT3 and Hb was strongly positive (p < 0.001) in demented as well as in nondemented patients, particularly in males. The concentration of fT4 was positively correlated to Hb in demented males (p < 0.001), whereas TSH concentration was positively correlated to Hb in demented females (p < 0.05). The results show that TSH is not sufficient as the sole screening assay for evaluation of possible thyroid dysfunction in psychogeriatric patients. In addition, central (hypothalamic?) hypothyroidism may be present in a substantial amount of psychogeriatric patients, as we found an adequate TSH response to exogenous thyrotropin-releasing hormone (TRH) also in patients with decreased fT3/fT4 and no signs of non thyroid diseases. Furthermore, there was an apparent lack of correlation between thyroid hormone levels and dementia (or subgroups of dementia), even though thyroid hormone abnormalities seemed to be rather common in frontotemporal dementia (38%) and non specified dementia (36%).

A neuroendocrine overview of aging

The neuroendocrine changes associated with aging are numerous, and tend to vary quantitatively, if not qualitatively between species. The extent to which neuroendocrine dysfunction contributes to the undesirable features of aging remains to be fully determined. Nevertheless, although the aging process itself may not have a neuroendocrine basis, identification and correction of the associated neuroendocrine dysfunction may be important in enhancing the quality of life during this period.

Thyroid disease in the elderly

Thyroid disease in the aged, both hypothyroidism and hyperthyroidism, may be subtle or may be present with no clinical symptoms and signs, and is therefore difficult to diagnose on the basis of clinical evaluation. The help of the laboratory is essential in making the diagnosis of disease of the thyroid. Therapeutic strategies are different in the aged than in the younger adult with thyroid disease. It is essential for geriatricians, and all clinicians who care for the elderly, to have a solid understanding of thyroid function and dysfunctions in this group of patients so that they diagnose diseases of the thyroid correctly and treat them appropriately.

Thyroid disease in the elderly

As people in our society live longer, it is important for clinicians to know more about problems commonly seen in the elderly. Thyroid problems are especially important to understand because they do occur with rather high frequency, and their mode of presentation is frequently different from those seen in younger patients. The thyroid gland does undergo certain age-related changes in anatomy and physiology, but overall the thyroid is able to produce a normal amount of thyroid hormone throughout the years. Hypothyroidism is frequently difficult to diagnose in the elderly, because many of the symptoms of hypothyroidism can easily be confused with symptoms of aging. When hypothyroidism is looked for in large population studies of elderly patients, the incidence varies from 1% to as high as 17%, with women being more commonly affected than men, and subclinical hypothyroidism being more common than overt hypothyroidism. Virtually all cases of hypothyroidism are due to autoimmune thyroid disease, with most patients having measurable titers of thyroid autoantibodies. The therapy of hypothyroidism is done with extreme caution, as older patients are frequently very sensitive to the effects of excess thyroid hormone. In addition, the metabolism of thyroid hormone slows down with age, making the full replacement dose much less in an older patient than in a younger one. Hyperthyroidism is also quite common, occurring in from 0.5% to 3% of all elderly patients. The presentation is frequently atypical, as patients often lack the hyperdynamic symptomatology and instead have a more sedated, apathetic presentation. Weight loss and cardiac symptoms frequently predominate, and the presence of a goiter is frequently absent, making the diagnosis less obvious than in a younger patient. Therapy is usually radioactive iodine, after an adequate course of antithyroid drugs, to render the patient euthyroid. Thyroid nodules do occur with increasing frequency in the elderly, but most of them are not malignant. Fine-needle aspiration for cytology is very helpful in determining which patients should be referred for surgery. Well-differentiated cancers do predominate, but their course is frequently less predictable than in younger patients. Lymphoma of the thyroid and undifferentiated cancers do occur with increasing frequency in the elderly. Multinodular goiter, usually of longstanding, is frequently seen in elderly patients, and thyroid hormone suppressive therapy not only is not indicated but may contribute to exogenous hyperthyroidism.

Hypothyroidism in the elderly

This review describes the changes in thyroid physiology with aging and notes that normal thyroid status is maintained with advanced age. The increased frequency of hypothyroidism in the elderly is stressed with consideration given to the multiple possible etiologies. The natural history of hypothyroidism is considered as evidenced by the various hormonal abnormalities in subjects with autoimmune thyroiditis and patients previously treated with radioiodine. The potential significance of subclinical hypothyroidism is discussed in regard to nonspecific symptoms and subtle cardiovascular manifestations. The problems in the clinical and laboratory diagnosis of hypothyroidism are reviewed with emphasis on the high index of suspicion and cautious interpretation of plasma thyroxine and thyroid-stimulating hormone (TSH) levels required. Finally the potential problems in treating hypothyroidism in the elderly are discussed in regard to dosage requirements and the need to avoid subclinical hypothyroidism by using the ultrasensitive TSH assay.

Relationship of age to TSH response to TRH in depressed men

The relationship between age and TSH response to TRH was studied in 40 men with unipolar major depressive disorder (range 24-65 years, mean 44.7 years) and 36 healthy male volunteers of similar ages. Both groups were subdivided into younger and older than 40 years of age. "Blunted" TSH response to TRH was observed in 58% of depressed men and in 28% of controls, using a dTSH maximum of less than or equal to 6 microU/ml as a cut-off criterion. Older healthy men had a higher blunting rate (40%) than the younger group (19%). In depressed patients, by contrast, the blunting rate was 50% in the older group and 65% in the younger group. Higher mean maximum dTSH, higher basal TSH and lower mean circulating FT4 levels were also noted in older depressed men, suggestive of a subtle thyroid subsensitivity to TSH stimulation and subclinical primary hypothyroidism that may have contributed to the depression. Age is known to be a confounder of TRH test results. There may be a subset of depressed patients over 40 where the confounding effect of age is associated with an exaggerated, rather than decreased TSH response to TRH.

Iodide administration enhances thyrotrophin responsiveness to thyrotrophin-releasing hormone during fasting: evidence for normal pituitary feedback regulation

Short-term fasting in humans is associated with diminished delta TSH to TRH. The purposes of the present study were to reassess basal TSH levels and TRH responsiveness during fasting utilizing a sensitive radioimmunoassay (RIA: sensitivity 0.3 microU/ml; normal range 0.66-2.98 microU/ml) and to determine if normal feedback regulation is maintained during the fasting state. Eight control subjects (C) and six iodide-treated (I) subjects (262 mg/d) were studied in the fed state and on day 10 of fasting. T3, T4, and TSH were measured by RIA, and free T4 and free T3 by equilibrium dialysis. Basal serum TSH levels in the control group were 2.0 +/- 0.3 microU/ml (mean +/- SEM) in the fed state and increased to 14.7 +/- 3.5 microU/ml 20 min after TRH administration. The fasting basal TSH level of 1.6 +/- 0.3 microU/ml was significantly decreased (P less than 0.01) compared to control, as was the level of 8.8 +/- 2.3 microU/ml (P less than 0.01) obtained 20 min after TRH. In the iodide-treated group the basal TSH level was 1.4 +/- 0.2 microU/ml during feeding which increased (P less than 0.025) to 2.9 +/- 0.7 microU/ml during fasting; the TSH value 20 min after TRH was 12.6 +/- 2.5 microU/ml while feeding and 17.3 +/- 2.9 microU/ml while fasting. Free and total T3 decreased during fasting in both groups. Total T4 was unchanged between the fed and fasted periods in the two groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Thyrotoxicosis in the very old

The records of 25 patients older than 75 years of age with the diagnosis of hyperthyroidism were reviewed. The mean age of the group (22 women and three men) was 81.5 years, the eldest being 95 years old. Twenty-one patients had Graves' disease, three had multinodular goiter, and one had toxic adenoma. Major presenting symptoms included weight loss (44 percent), palpitations (36 percent), and weakness (32 percent). The average number of thyrotoxic symptoms was only two per patient. Two patients were asymptomatic. Clinical signs included fine skin (40 percent), tremor (36 percent), atrial fibrillation (32 percent), and tachycardia (28 percent). The thyroid was palpable in only three patients with Graves' disease. Mean blood thyroxine level was 15.6 micrograms/dl (range, 11.5 to 24); blood triiodothyronine level was elevated in only half of the patients. One patient had triiodothyronine toxicosis. Mean 24-hour radioiodine uptake was 52 percent. Five patients had normal uptake. No correlation could be established between age, clinical symptoms, signs, and hormone blood levels. Because signs and symptoms of hyperthyroidism in the very old may be too subtle for clinical diagnosis, all elderly subjects should have periodic screening of blood thyroxine levels.

Effects of nonthyroidal illness on thyroid function

Despite the absence of thyroid disease, patients with nonthyroidal illness frequently have changes in serum thyroid hormone measurements that may suggest either hypothyroidism or hyperthyroidism. Serum T3 levels are frequently decreased mainly because of a decrease in the rate of T3 production from T4. The free T3 concentration may be either normal or reduced as well. The binding of T4 and T3 by the serum-binding proteins is almost always impaired, resulting in an increase in the dialyzable fraction (free) fraction. This is due to a decrease in the concentration of thyroxine-binding proteins and the presence of circulating inhibitors of binding. If serum T4 concentration remains within the normal range, the free T4 concentration can be increased. However, serum T4 is frequently decreased in patients with chronic and/or severe illness. The decrease in serum T4 in these patients, combined with an increase in the dialyzable fraction, results in normal free T4. In patients who are critically ill, none of the available methods for measurement of free T4 may give results that accurately reflect the euthyroid state. Since T3 is the major active thyroid hormone, it is surprising that patients with decreased serum T3 do not appear hypothyroid. The decrease in serum T3 is probably an adaptive change to nonthyroidal illness, which at least enables the sick patient to conserve protein. The clinical impression of euthyroidism is supported by the finding of a normal serum TSH level in most patients. Although TSH regulation may not be entirely normal in patients with nonthyroidal disease, it is likely that serum TSH will be increased in most sick patients who also have significant thyroid failure. The normal clinical findings in patients with decreased serum T3 may result from an augmentation of those biologic responses associated with the clinical manifestations of the euthyroid state. Several animal models of nonthyroidal disease or starvation show that cells have the ability to modulate some biologic responses to thyroid hormone. Further study should elucidate the mechanisms underlying these changes. This article has emphasized that no single laboratory measurement may reliably predict the thyroid state in patients with nonthyroidal disease. This fact emphasizes the need for careful clinical evaluation of these patients and judicious use of laboratory tests. Because the changes in thyroid hormone metabolism that occur in nonthyroidal disease probably represent adaptive changes to the illness, treatment with L-thyroxine to restore serum thyroid concentrations to the normal range is not indicated.

Thyroxine supplementation. Method for the prevention of clinical hypothyroidism

Elevation in the blood level of thyroid-stimulating hormone (thyrotropin) is the earliest and most sensitive manifestation of thyroidal failure, and is detectable in clinically healthy and apparently euthyroid persons. Oral thyroxine supplementation designed to titrate thyrotropin back to normal levels, and readjustment of the supplementary dose as failure of the gland progresses and thyrotropin level rises again, may prevent the emergence of clinical hypothyroidism.

Response of hepatic mitochondrial alpha-glycerophosphate dehydrogenase and malic enzyme to constant infusions of L-triiodothyronine in rats bearing the Walker 256 carcinoma. Evidence for divergent postreceptor regulation of the thyroid hormone response

To characterize the hepatic response to L-triiodothyronine (T3) in an experimental nonthyroidal disease, we determined the activity of hepatic mitochondrial alpha-glycerophosphate dehydrogenase (alpha-GPD) and cytosol malic enzyme (ME) as a function of the saturation of the nuclear T3 receptor during constant T3 infusions in rats bearing the Walker 256 carcinoma. Groups of control and tumor-bearing rats were infused by minipumps (Alza Corp., Palo Alto, CA) with vehicle, 1.2 or 4.5 micrograms T3/100 body wt per day for 3 d. The range for serum T3 was 47.2 +/- 4.1 to 165 +/- 17.3 ng/dl for the control rats and 13.2 +/- 1.3 to 135 +/- 14.3 ng/dl for the tumor-bearing rats. Nuclear T3 receptor concentration was between 0.41 +/- 0.06 and 0.47 +/- 0.02 ng/mg DNA in control rats and was decreased in tumor-bearing rats to between 0.23 +/- 0.03 and 0.26 +/- 0.03 ng/mg DNA. Nuclear T3 receptor concentrations were not influenced by the T3 infusions. Specifically bound nuclear T3, determined by radioimmunoassay of extracts of isolated nuclei, was decreased nearly 50% in the tumor-bearing rats. However, the calculated percentage saturation of the T3 nuclear receptor remained similar in control and tumor-bearing rats at each level of T3 infusion. Dose-response curves for alpha-GPD and ME were curvilinear and showed an exponential increase in enzyme activity with progressive receptor saturation. In tumor-bearing rats, the activity curves or calculated appearance rate curves for alpha-GPD were shifted significantly upward and to the left, indicating greater sensitivity to T3, and those of ME were shifted downward and to the right, indicating decreased responsiveness to T3. Our findings suggest that cellular factors result in postreceptor amplification of the alpha-GPD response and diminution of the ME response to T3 in tumor-bearing rats. Augmentation of the alpha-GPD response may be a prototype for other hormonal responses that enable the tumor-bearing rat to maintain an apparent euthyroid state in association with decreased serum T3.