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

Abstract

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.

The aging process and function of endocrine glands

“…Autoimmune symptoms, e.g Graves' opthalmopathy or pretibial oedema, are much less frequent. Other symptoms include: loss of appetite, major loss of body mass, diarrhoea, apathy, somnolence, nervous hypersensitivity, despondency or depressive disorders (socalled apathetic thyrotoxicosis) The clinical image frequently leads to misdiagnosing the disease as a cancer [21,51]. A reduced TSH blood level coexists with an elevated fT4 level, accompanied by an unaltered fT3 level, which is caused by the age-associated impairment of fT4 to fT3 peripheral conversion.…”

“…However, sometimes so-called T3 toxicosis develops with a normal, maintained T4 level. Hyperthyroidism can be associated with a reduced TSH level, normal fT4 level and an elevated level of sex hormones binding globulin (SHBG) [21,51]. The epidemiological distribution of hyperthyroidism in the elderly depends on the iodine supply in a specific population.…”

“…The symptoms of thyrotoxicosis can be also observed after overdosing levothyroxine. Occasionally, hyperthyreosis is caused by thyroid cancer or excessive TSH secretion by a pituitary adenoma or when the pituitary gland is resistant to thyroid hormones [21,51]. It has been shown that subclinical hyperthyroidism in elderly people increases the risk of coronary heart disease and mortality rates [45], which correlates with a three-fold higher risk of atrial fibrillations [53], Wykazano, że ryzyko choroby niedokrwiennej serca i zgonu u ludzi z subkliniczną niedoczynnością tarczycy powyżej 65 r.ż.…”

“…apatyczna czy prostracyjna tyreotoksykoza). Obraz kliniczny jest nierzadko przyczyną mylnego rozpoznawania choroby nowotworowej [21,51]. Obniżone stężenie TSH we krwi współistnieje z podwyższonym poziomem fT4, któremu towarzyszy niezmieniony poziom fT3, co ma związek z zaburzoną z powodu wieku konwersją obwodową fT4 do fT3.…”

“…T3-toksykoza z prawidłowym poziomem T4. W nadczynności tarczycy należy oczekiwać obniżonego poziomu TSH, prawidłowego fT4 oraz podwyższonego poziomu globuliny wiążącej hormony płciowe (SHBG) [21,51]. Epidemiologia nadczynności tarczycy w wieku podeszłym zależy od podaży jodu w danej populacji.…”