The thyroid and age

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.

The hepatic transcellular transport of 3,5,3'-triiodothyronine is reduced in aged rats

The mechanisms of age-related reduction in tissue-responsiveness to thyroid hormones is not clear. Since the first step in hormone action is the transport of the hormone from plasma to the tissues, tissue uptake of T3 was determined in three groups of male Fischer 344 rats: young (6 months old), aged (24-26 months old) and young rats pair-fed with aged rats. At steady state, total tissue uptake of T3 in the liver, heart and rectus abdominis muscle was reduced in aged rats while T3 uptake by cerebral tissues, femoris and soleus muscles was not altered with age. The subcellular distribution of T3 in the liver was determined and the binding power of cytosol and plasma was measured by equilibrium dialysis. In vitro saturation techniques provided estimates for the affinity (Ka) and binding capacity for L- and D-T3 of isolated hepatic nuclei at 37 degrees C. The plasma concentration of T3 was determined with radioimmunoassay. From these parameters the free cytosolic to plasma T3 ratio (Fc/Fp) and free nuclear to cytosolic ratio (Fn/Fc) were calculated. The Fc/Fp for L-T3 was significantly reduced in aged rats (2.34 +/- 0.15) compared to young rats (4.33 +/- 0.16) and young rats pair-fed with old (3.55 +/- 0.46). The Fc/Fp for D-T3 were 6.2 +/- 0.39, 24.3 +/- 2.3 and 10.1 +/- 1.5, respectively. The affinity constant (Ka) and the maximum binding capacity measured in isolated hepatic nuclei were not different in the three groups of rats. However, the nuclear uptake of L-T3 (T3N/P: percentage of dose per mg DNA per percentage of dose per ml plasma) was significantly reduced in aged rats (0.29 +/- 0.03) and young rats pair-fed with old (0.32 +/- 0.02) compared to young rats fed ad libitum (0.44 +/- 0.02). The corresponding values of D-T3 were 0.09 +/- 0.007, 0.13 +/- 0.006 and 0.22 +/- 0.01, respectively. The Fn/Fc of L- and D-T3 was not altered in aged rats or young rats pair fed with old. The liver uptake index (Ul) of L-[125I]T3 was determined in vivo with single injection tissue sampling technique. The liver uptake index in old rats (73.3 +/- 9.9%) was significantly reduced compared to young rats (107.5 +/- 9.4%). These results indicate that (1) cellular uptake of T3 is reduced in aged rat liver. These changes may be in part secondary to age-related reduction in food intake.(ABSTRACT TRUNCATED AT 400 WORDS)

Age-induced decreases in the messenger RNA coding for the sarcoplasmic reticulum Ca2(+)-ATPase of the rat heart

Age-associated slowing of cardiac relaxation related to the decline in the Ca2+ pump function of cardiac sarcoplasmic reticulum (SR) has been previously described. It is unclear if the decreased Ca2+ pump function results from a lower amount of Ca2(+)-ATPase protein or a decreased pumping activity of the enzyme. To determine if these alterations could be mediated by changes in the amount of the protein itself, the level of the messenger RNA (mRNA) coding for the Ca2(+)-ATPase of the SR of Fischer rat hearts (4- and 30-month-old rats) were quantitated with a Northern blotting technique. We observed that the levels of SR Ca2(+)-ATPase mRNA were 60% lower in old rats as compared with young rats, suggesting that a quantitative reduction in the levels of the corresponding protein could occur during aging to explain the delayed diastolic relaxation documented in old animals as opposed to a change in the specific activity of this enzyme. The thyroid hormone responsiveness of SR Ca2(+)-ATPase mRNA has been previously established. We have found in this study that the thyroxine levels were consistently lower in old rats; however, this difference was relatively small (4.3 +/- 0.7 and 3.1 +/- 0.8 micrograms/dl [mean +/- SD), respectively, in young and old rats). In addition, no age-induced decrease in 3,5,3'-triiodothyronine levels was observed, suggesting that the aging process itself may be responsible for the changes in SR Ca2(+)-ATPase mRNA levels.

Peculiarities of neural regulation of the thyroid, adrenocortical and testicular functions in old age

The age changes of neural control over the function of the thyroid, adrenal cortex and testicles were examined in adult (6 months) and old (28 months) male rats. In old age, there was a weakening of adrenergic control of thyroidogenesis, alpha-adrenergic and M-cholinergic control of glucocorticoid function of the adrenal cortex and reduction of adrenergic and M-cholinergic influences in the regulation of steroidogenic function of the testicles.

Age-related differences in body weight loss in response to altered thyroidal status

To determine whether age-related differences in body weight loss in hyperthyroidism could be related to caloric intake, the body weight and food consumption of Fischer 344 male rats were monitored every other day for four weeks. Six-month-old (young) rats were compared to 16-month-old rats (intermediate age) and 25-month-old (aged) rats. Hypothyroidism was induced with 0.025% methimazole in the drinking water for four weeks. Hyperthyroidism was induced with triiodothyronine (T3) injections (15 micrograms/100 g body weight i.p.) for the last 10 days of observation. A group of young rats pair fed with aged rats was included as a control group. The body weight changes of aged rats were similar to hypothyroid young rats. An index of T3 catabolic effect was calculated based on the net weight loss and food intake. This index was not different in aged rats compared to young rats. The apparent hypersensitivity of aged rats to T3 as evidenced by excessive weight loss could totally be attributed to decreased caloric intake. It is concluded that aged rats compared to the young are not more sensitive to the overall catabolic effects of thyroid hormones.

Influence of sex and age on T3 receptors and T3 concentration in the pituitary gland of the rat: consequences on TSH secretion

A reduced secretion of thyroid hormones with age has been documented in humans and animals with no substantial increase in TSH secretion, which may be indicative of an age-related impairment of the pituitary sensitivity to the negative control exerted by thyroid hormones. We have evaluated in rats the influence of sex and age on pituitary T3 nuclear receptors--known to be determinant in the regulation of TSH secretion--as well as on T3 concentration in the pituitary gland. As regards sex, the density of T3 receptors and the concentration of T3 in pituitary gland and plasma were greater in females than in males whereas pituitary and plasma TSH concentrations were less. As for age, the density of T3 receptors was greater in old male rats than in young ones with no changes in pituitary T3 and plasma TSH concentrations. In old female rats in contrast, there was no significant increase in T3 receptors but pituitary T3 was less and plasma TSH greater than in young female rats. In both sexes plasma thyroid hormones and pituitary TSH were reduced with age whereas TSH response to TRH was not altered. These results illustrate sex and age differences in pituitary T3 receptors and pituitary T3 concentration as well as in TSH secretion. In young animals of both sexes an inverse correlation is observed between the density of pituitary T3 receptors and plasma TSH. In contrast, in old animals the absence of this correlation is suggestive of an age-related impairment of T3 action on the thyrotrophs or of changes pertaining to other factors modulating TSH secretion.

Endocrine changes associated with the human aging process: III. Effect of age on the number of calcitonin immunoreactive cells in the thyroid gland

The thyroid glands obtained at autopsy from 60 patients ranging in age from 16 to 89 years were immunostained for calcitonin (CT) by the peroxidase-antiperoxidase procedure. The numbers of CT immunoreactive cells identified were 0.99 +/- 1.07/mm2 in the young patients (16 to 39 years of age), 0.99 +/- 1.46/mm2 in the middle-aged (40 to 59 years of age), and 2.97 +/- 3.69/mm2 in the elderly (60 years of age and older). The results were not statistically significant because of the large standard deviation. The CT immunoreactive cells tended to aggregate in clusters in a pattern similar to that seen in C-cell nodules in older persons.

Regulation of NAD- and NADP-linked isocitrate dehydrogenase by thyroxine in the brain and liver of female rats of various ages

The specific activity of NAD- and NADP-linked isocitrate dehydrogenase and their regulation by thyroxine in the brain and liver of female rats of various ages were studied with the ultimate goal of better understanding the decreased physiological functioning of the brain and liver during old age. Both thyroidectomy and thyroxine treatment have differential age-dependent effects on the activities of these enzymes in both tissues. The activity of NAD-ICDH decreases whereas both cytoplasmic and mitochondrial NADP-ICDH increase simultaneously following thyroidectomy. Thyroxine administration induces NAD-ICDH and depresses NADP-ICDH. The degree of induction and/or repression is lowest in old rats. These effects of thyroxine are actinomycin D sensitive in both the tissues of rats.

Thyroxine deiodination, cytoplasmic distribution and nuclear binding of thyroxine and triiodothyronine in liver and brain of young and aged rats

This study examines (a) the effects of aging on plasma thyroid hormone concentration and (b) in vivo binding and cytoplasmic distribution of thyroid hormones as well as the conversion of thyroxine (T4) to triiodothyronine (T3) in liver and cerebral hemispheric tissue. The results show that (a) in male Long-Evans rats aging decreases plasma T4 concentration but does not affect plasma T3 concentration and (b) the in vivo nuclear T3 binding does not change significantly. However, nuclear T3 binding derived from T4 is decreased as a consequence of reduced T4 to T3 conversion in both tissues. The nuclear T4 binding is also depressed, perhaps due to the decrease in the T4 of the protein free cytoplasmic compartment. Aging was also found to change protein free and protein bound cytoplasmic distribution of T4. That is, an increase was observed in protein bound cytoplasmic T4 and a decrease in the protein free cytoplasmic T4 of both tissues. These results indicate an overall alteration in thyroid hormone production and peripheral tissue binding and processing of thyroid hormones with a consequent suboptimal thyroid state with aging.

Axoplasmic transport of substances in motoneuronal axons of the spinal cord in old age

Seven to eight microliters of aqueous solution of L-[14C]Leucine (spec. act. 339 mCi/mmol) were introduced in the zone of ventral horn (L5-L6) of the spinal cord of adult (8-12 months) and old (26-28 months) rats. The radioactivity of various parts of the corresponding ventral roots was measured 1-2.5 h thereafter. Labelled substances (including protein) were found to migrate with fast flow in adult rats at the rate of 408 +/- 10.9 and 380 +/- 22 mm/24 h, respectively, as compared with 217 +/- 11.3 and 200 +/- 40 mm/24 h in old rats. The axoplasmic flow slows down in old rats with the increase of distance from the neuronal body. Uncoupling of oxidation and phosphorylation by the administration of 2,4-dinitrophenol, inhibition of glycolysis with NaF, hypoxemia produce more marked deceleration of axoplasmic flow in old rats, while small doses of NaF accelerate the flow, which correlates with the rise of cAMP in ventral roots. Sex steroids accelerate significantly the rate of axoplasmic flow. There is a marked increase in the rate due to the administration of estradiol dipropionate in old rats and due to testosterone propionate in adult animals. Changes in resting membrane potential and direct excitability thresholds of some muscles following colchicine blockade of axoplasmic transport are less marked in old rats that evidences for the weakening of neurotrophic control in old age.

Reduction due to ageing of TSH-stimulated thyroid hormone release in the cream hamster

The in vitro secretion by thyroid lobes from young (2-month-old) and from "aged" (more than 21 months old) male cream hamsters was estimated by measuring the butanol-extractable 125I (BE-125I) released into the medium after 2 and 4 h of incubation in the presence or absence of thyroid stimulating hormone (TSH). Lobes were fixed at the end of incubation for examination by both light and electron microscopy. Other lobes from both groups were collected for estimation of cyclic AMP contents. BE-125I release by lobes from aged animals after TSH stimulation was significantly lower than that of young animals. Similarly, pseudopods resorbing colloid, the morphological markers of secretion, were much less prominent in the aged group than in the young group. As this observation and the reduced BE-125I release contrast with an equivalent increase of cyclic AMP in both the young and aged groups, it is suggested that the age-dependent defect resulting in reduced hormone secretion is located at a step beyond the generation of cyclic AMP in the cascade of events occurring between binding of TSH to plasma membrane receptors on the target cell and hormone release.

Age-related metabolic modifications in male F344 rats

Basal metabolic rate, basal colonic temperature, and response to intraperitoneal injection of tryamine were examined in three age groups: young (3 months), adult (12 months), and old (24 months) of male Fischer 344 rats. Basal metabolic rate of sedentary male F344 rats decreased significantly between 3 months (21.5 ml O2 X kg-1 X min-1) and 1 year (19.4 ml) of age and continued to decrease in the oldest animals (19.0 ml). The decrease between 1 and 2 years was not statistically significant. Basal colonic temperature showed a step-wise significant increase with age. Mean colonic temperature was 36.7 degrees C at 3 months, 37 degrees C at 1 year, and 37.5 degrees C at 2 years. Higher optimal doses of tyramine were required for 1- and 2-year-olds than for 3-month-olds. The optimal dose was 2 mg X kg-1 for 3-month-olds and 5 mg X kg-1 for 1- and 2-year-olds. Although the optimal dosages were the same for the 1- and 2-year-olds, 2-year-olds had lower peak response and the response was slower than for the 1-year-olds.

Serum thyroxine, triiodothyronine, and TSH levels and TSH release after TRH in aging male and female rats

Serum levels of TSH, T3 and T4, pituitary responsiveness to TRH, and T3-resin uptake were examined in aging male and female rats. Despite normal serum TSH levels, serum T3 and T4 levels were significantly reduced in old male rats (19-26 months of age) when compared to young (120-150 days of age) and middle-aged males (12-1- months of age). In old female rats, serum TSH and T3 levels were not significantly (p greater than 0.05) different from those of young or middle-aged females. Serum T4 levels were significantly lower in old females when compared to younger rats. The T3-resin uptake of old rats was not significantly different from the value of young rats. Serum T4 levels, 3 hours after TRH induced TSH release, were significantly depressed in old male and female rats. Pituitary sensitivity to TRH was unaltered in old rats. These findings suggest that thyroidal release of T4 and T3 is depressed, and that the hypothalamic-pituitary axis is insensitive to diminished thyroid hormone feedback in old rats.

Effect of aging on the morphology and function of the thyroid gland of the cream hamster. Further evidence for two different mechanisms of hormone secretion

The effect of aging on the morphology and function of the thyroid gland of the cream hamster was studied by light and electron microscopy coupled with autoradiography or histochemistry. Morphologically, aging induces an accumulation of lysosomal dense bodies and a loss of the phagocytosis of colloid droplets after stimulation with TSH. Iodine uptake and organification occur normally and thyroglobulin synthesis, estimated by autoradiography with 3H-leucine, is not different from that observed in young animals. The basal T4 and T3 plasma levels are lower in the old animals. A low iodine diet administered for several months prevents the age related accumulation of lysosomal dense bodies. Hormone secretion seems to proceed by two different mechanisms; phagocytosis of colloid droplets, the classical mechanism that decreases with age, and an additional mechanism, probably micropinocytosis, that is maintained during the whole lifespan.

Changes in pituitary and thyroid function with increasing age in young male rats

As the age of young adult male rats increased from 30 to 150 days, the serum thyroxine (T4) decreased by 50% and the serum thyroid-stimulating hormone (TSH) increased by 250%. There was no change in the serum triiodothyronine (T3). The increment in serum TSH after injection of thyrotropin-releasing hormone (TRH) was not significantly different at any of the ages studied, but the old animals had significantly lower increments in serum T4 and T3 after subcutaneous administration of bovine TSH. Despite a higher basal serum TSH, the older rats had a lesser increase in serum TSH after thyroidectomy or propylthiouracil. Thus, 1) there is a progressive decline in intrinsic thyroid function between 30 and 150 days of age in male rats, and 2) pituitary TSH response to fall in serum concentration of thyroid hormones is also decreased with age.

Influence of the thyroid gland on ovarian function in the aging rat

The effects of underfeeding and manipulation of the thyroid axis on ovarian function were determined in young and old rats. The depressant effect of reduced food intake on ovarian cycling in young females was potentiated by chemical thyroidectomy, while young anestrous, underfed rats cycled when their diet was supplemented with thyroid extract. These observations iindicate that cycling aberrations in underfed rats may occur secondarily to an altered thyroid state. To determine if thyroid state influences ovarian function in old animals, constant estrous (CE) rats were underfed or chemically thyroidectomized. All underfed rats eventually cycled, while the response to chemical thyroidectomy alone, though still effective, was less dramatic. Realimented CE animals eventually returned to a pattern of constant vaginal cornification. Underfeeding had no effect on ovarian function in old recurrently pseudopregnant females however, these rats responded to thyroid treatment with renewed cycling. 4-6 (YC) 10-12 (PEP) month old females entered a persistent vaginal estrous condition when fed low doses of thyroid extract with their ad libitum diet. When the thyroid supplemented diet was discontinued, YC females resumed regular cycling, whereas the vaginal smear in 40% of the PEP rats remained cornified. Cycling could be restored in YC-thyroid induced CE rats by electrochemical stimulation of the medial preoptic area. These date suggest that senile deviations from normal cycling in the aging reproductive system may be affected by alterations in the thyroid state.