In one study, serum markers for increased bone metabolism and turnover were evaluated in 36 cats that had elevated serum T4 concentrations and elevated serum ALP activities.7 Serum total calcium, ionized calcium, and phosphorus concentrations were measured. ALP isoenzymes were separated by agarose
gel electrophoresis, and the concentration of osteocalcin, a marker of increased bone turnover, was measured by radioimmunoassay.
The test results in the hyperthyroid cats were compared with those in healthy cats. All 36 hyperthyroid cats had markedly
increased ALP bone isoenzyme activity. Forty-four percent of the cats had increased osteocalcin concentrations. No correlation
was found among the magnitude of increase in ALP bone isoenzyme activity, osteocalcin, and serum T4 concentrations. Thirty-five percent of the cats had increased serum phosphorus concentrations. All the cats had total calcium
concentrations within the reference range, while 50% of the cats had decreased serum ionized calcium concentrations. This
study provided evidence that bone turnover is increased in hyperthyroid cats but that it is not clinically important because
the increased bone turnover does not result in clinical signs (e.g. pain, arthritis, fracture) as a result of osteoporosis. The finding of low serum ionized calcium concentrations in half of
the hyperthyroid cats is interesting, and this topic relates to the study described next.
Parathyroid hormone (PTH) and ionized calcium concentrations in 30 untreated hyperthyroid cats and 38 age-matched normal (control)
cats were evaluated.8 The hyperthyroid group had significantly lower blood ionized calcium and plasma creatinine concentrations and significantly
higher plasma phosphorus and PTH concentrations. Seventy-seven percent of the hyperthyroid cats had hyperparathyroidism, with
PTH concentrations up to 19 times the upper limit of the reference range. The authors stated that the etiology, significance,
and reversibility of hyperparathyroidism in hyperthyroid cats have yet to be determined but could play a role in these cats'
bone strengths and renal functions. Hyperthyroid cats may have decreased creatinine concentrations from either increased glomerular
filtration rates or decreased muscle mass, though that was not evaluated in this study. The effects of thyrotoxicosis on renal
function are discussed in the next article.
The effect of hyperthyroidism on clinical and laboratory findings hasn't been fully elucidated. Because many hyperthyroid
cats are geriatric and are likely to have concurrent disease, it remains important to thoroughly investigate these interrelationships.
Two recent studies evaluated the effects of hyperthyroidism on laboratory assessment of glycemic control (by measuring serum
fructosamine concentrations) in cats with concurrent diabetes mellitus.
In cats, serum fructosamine concentrations reflect the mean blood glucose concentration of the preceding one or two weeks.
However, fructosamine concentrations are affected by the concentration and metabolism of serum proteins. Hyperthyroidism can
profoundly increase protein metabolism and, therefore, possibly affect serum fructosamine concentrations. In one study, hyperthyroidism
was diagnosed in 22 cats, ranging in age from 8 to 20 years, based on clinical signs, detection of a palpable thyroid gland,
and a serum total T4 concentration greater than 45 nmol/L.9 Blood glucose, total protein, and albumin concentrations were within reference ranges. The serum fructosamine concentrations
of the 22 hyperthyroid cats were compared with those of 42 healthy control cats, 10 newly diagnosed diabetic cats, and nine
cats with hypoproteinemia. Serum total T4 concentrations ranged from 46 to 475 nmol/L (median µ 86 nmol/L). Hyperthyroid cats had serum fructosamine concentrations
between 154 and 267 µmol/L (median µ 198 µmol/L), significantly less than those in healthy cats. Serum fructosamine concentrations
in cats with hypoproteinemia ranged from 124 to 254 µmol/L (median µ 174 µmol/L; reference range = 175 to 400 µmol/L) and
were significantly less than those in healthy cats. Serum fructosamine concentrations did not differ between hypoproteinemic
and hyperthyroid cats. In hypoproteinemic cats, concentrations of serum total protein and albumin were significantly lower
than those in hyperthyroid cats, while blood glucose concentrations did not differ between the two groups of cats. Serum fructosamine
concentrations in diabetic cats were significantly increased compared with those in healthy cats, hypoproteinemic cats, and
hyperthyroid cats. After two weeks of carbimazole treatment (discussed in the next article) in six of the hyperthyroid cats,
serum fructosamine concentrations were not significantly different from the initial concentrations. After six weeks of carbimazole
treatment, serum fructosamine concentrations were significantly higher than the initial concentrations. Serum total T4 concentrations were significantly decreased both two and six weeks after initiating treatment. The authors concluded that
serum fructosamine concentrations are lower in cats with hyperthyroidism independent of blood glucose concentrations. In the
clinical setting, serum fructosamine concentrations should not be used to initially diagnose or assess the adequacy of diabetic
control in cats with concurrent hyperthyroidism in which the hyperthyroidism has not been controlled for at least six weeks.