Serum thyroxine and thyroid‐stimulating hormone concentration in hyperthyroid cats that develop azotaemia after radioiodine therapy

Abstract

OBJECTIVES : The objectives of this study were to determine which serum thyroid hormone test best identifies iatrogenic hypothyroidism in cats that develop azotaemia after radioiodine treatment and to determine which thyroid test best differentiates these azotaemic, hypothyroid cats from azotaemic, radioiodine-treated euthyroid cats, as well as from azotaemic cats with chronic kidney disease and no history of thyroid disease. M ATERIALS AND M ETHODS : A total of 42 hyperthyroid cats that developed azotaemia (serum creatinine ê220 µmol/L) after radioiodine treatment had serum concentrations of thyroxine and free thyroxine by dialysis and thyroid -stimulating hormone measured at 3, 6 and 12 months. Iatrogenic hypothyroidism was confirmed (n=28) or excluded (n=14) on the basis of thyroid scintigraphy. A total of 14 cats with chronic kidney disease and 166 clinically normal cats underwent similar serum thyroid testing and scintigraphy.R ESULTS : Concentrations of thyroxine and free thyroxine were lower and thyroid-stimulating hormone higher in hypothyroid cats than in all three groups of euthyroid cats (P<0·0001). Of the hypothyroid cats, thyroxine and free thyroxine concentrations were low in 15 (53·6%) and seven (25%), respectively. Low serum thyroxine and free thyroxine concentrations were also detected in seven (50%) and two (14·3%) of the cats with chronic kidney disease. Thyroid-stimulating hormone concentrations were elevated in all hypothyroid cats but remained within the reference interval in all three groups of euthyroid cats.Serum thyroid -stimulating hormone had a higher test sensitivity and specificity than either thyroxine or free thyroxine concentration. C LINICAL SIGNIFICANCE : The finding of high serum thyroid-stimulating hormone concentrations best identifies feline iatrogenic hypothyroidism and differentiates it from non-thyroidal illness syndrome in cats that develop azotaemia after treatment. ttp://www.bsava.com/ M. E. Peterson et al.

Evaluation of Serum Symmetric Dimethylarginine Concentration as a Marker for Masked Chronic Kidney Disease in Cats With Hyperthyroidism

“…Our study confirmed that cats with iatrogenic hypothyroidism are more likely to become azotemic than cats that remain euthyroid . Additionally, the proportion of cats that develop azotemia after treatment for hyperthyroidism depends, at least in part, on the degree to which the hyperthyroidism is controlled (ie, how low the serum T 4 falls) .…”

“…Direct GFR measurements have not been reported in hypothyroid cats—neither before nor after levothyroxine replacement—but those studies are needed. We recently reported, however, that levothyroxine can lower serum creatinine concentrations in 131 I‐induced (iatrogenic) hypothyroid cats with moderate azotemia, but we know of no studies on the effect of levothyroxine on serum SDMA concentrations in hypothyroid cats—additional studies are again needed.…”

“…28 Our study confirmed that cats with iatrogenic hypothyroidism are more likely to become azotemic than cats that remain euthyroid. 11,14 Additionally, the proportion of cats that develop azotemia after treatment for hyperthyroidism depends, at least in part, on the degree to which the hyperthyroidism is controlled (ie, how low the serum T 4 falls). 11,14 Importantly, azotemia is more likely to develop in cats with overt hypothyroidism than in cats with subclinical hypothyroidism or in 131 I-treated cats that become euthyroid.…”

“…11,14 Additionally, the proportion of cats that develop azotemia after treatment for hyperthyroidism depends, at least in part, on the degree to which the hyperthyroidism is controlled (ie, how low the serum T 4 falls). 11,14 Importantly, azotemia is more likely to develop in cats with overt hypothyroidism than in cats with subclinical hypothyroidism or in 131 I-treated cats that become euthyroid. 11,13,14,28 As in previous studies, these cats with overt hypothyroidism had a higher prevalence of azotemia (80%) than did the cats with milder, subclinical hypothyroidism (31%).…”

“…Currently, no clinical test can reliably predict which hyperthyroid cats have concurrent, masked CKD . Identifying these cats could influence the choice of treatment for hyperthyroidism, as well as the care taken to minimize post‐treatment iatrogenic hypothyroidism, which can worsen existing azotemia and shorten survival time in cats with CKD . Predicting which untreated hyperthyroid cats have masked concurrent CKD might also allow clinicians to intervene earlier in the course of the disease.…”