In cats with concurrent hyperthyroidism and non-thyroidal illnesses such as chronic kidney disease, total thyroxine concentrations are often within the laboratory reference range. A recent published study determined total thyroxine (T4) , free thyroxine (FT4 and/or thyroid-stimulating hormone (TSH) concentrations in cats with mild chronic kidney disease.

T4, FT4ED, and TSH were measured in three groups. The hyperthyroidism + chronic kidney disease group (n=516) had chronic kidney disease and clinical signs compatible with hyperthyroidism but T4 concentration within the reference range. These cats were subsequently confirmed to be hyperthyroid. The chronic kidney disease-only group (n= 520) had chronic kidney disease but no signs of hyperthyroidism. The normal group (n=520) comprised of clinically healthy senior (> 8 years) cats.

In 4 of 20 euthyroid chronic kidney disease cats, FT4 concentrations were borderline or high (> 40 pmol/L). In the hyperthyroidism + chronic kidney disease group, FT4 was high in 15 of 16 cats, while TSH was low in all 16. Most hyperthyroidism + chronic kidney disease cats (14 of 16) had T4 > 2.3 µg/dL (> 30 nmol/L), whereas all the chronic kidney disease-only cats had T4 < 2.3 µg/dL (< 30 nmol/L).

Results demonstrated that when hyperthyroidism is suspected clinically in a patient with chronic kidney disease, the diagnosis should be pursued if the T4 concentration is in the middle or high end of the reference range.

This study also identified a decline in creatinine concentration that occurred during the onset of naturally occurring feline hyperthyroidism. Although an increase in creatinine concentration following the treatment of hyperthyroidism has been well documented, decrease in creatinine concentrations in cats with hyperthyroidism has not be shown previously in a clinical setting.

In general practice, where a cat may only be seen for a single examination, a high degree of clinical suspicion for senior cats with hyperthyroidism and concurrent chronic kidney disease must be maintained.

It has been suggested that a T4 > 2µg/dL (>25 nmol/L) can be considered as suspicious for hyperthyroidism in senior cats with concurrent illness. The combination of a high normal T4 with a high FT4 has been shown to be useful for the diagnosis of hyperthyroidism in cats with concurrent illness. This cut-off was supported by results of the present study.

FreeT4 concentrations are less often depressed by non-thyroidal illness and FT4 has been shown to be more sensitive than T4 as a single test for the diagnosis of hyperthyroidism in cats. However, FT4 can be elevated or borderline elevated in non-thyroidal illness in cats without evidence of thyroid disease, as was found in 6–20% of cats in the present study. For this reason (poor specificity) , FT4 should not be used alone for the diagnosis of hyperthyroidism in cats with chronic kidney disease or other non-thyroidal illness such as liver or gastrointestinal disease.

The combined measurement of T4 > 2.3 µg/dL (> 30 nmol/L) and FT4 > 40 pmol/L or with TSH < 0.03 ng/mL is of diagnostic value for the confirmation of hyperthyroidism in any cat with mild or previously diagnosed chronic kidney disease compatible clinical signs.

References: Wakeling, Moore, Elliott et al. J Sm An Pract 6:1-8, 2008.

Iodine excess causes alterations in thyroid activity, blocking both its characteristic functions and cell proliferation. Depending on the dose of iodine and on the previous conditions of the gland, iodine excess can have a goitrogenic effect and induce the blockade of hormone biosynthesis and secretion-provoking hypothyroidism.

Three groups of puppies younger than 3 months old were fed different diets: 1) a home-prepared diet (control group), 2) a commercial diet (containing 5.6 mg potassium iodide/kg dry food), and 3) a home-prepared diet supplemented with 5.6 mg potassium iodide/kg dry food. Thyroid volume was evaluated by sonography and by weight, histopathology, and morphometry, and thyroid hormones were measured (thyroid stimulating hormone [TSH] and thyroxine T4). Volume, weight, and diameter of the thyroid follicles were increased (p < 0.05) in the 2 iodine-supplemented groups as compared with the control group. TSH was increased (p < 0.05) and T4 was lower (p < 0.05) in groups with a high amount of iodine in diets than in the control group.

Conclusions and Clinical Relevance Increased dietary iodine alters thyroid morphology and function in puppies younger than 3 months old. Feeding excessive amounts of iodine in foods and dietary supplements (kelp, seaweed) could reduce thyroid function and perhaps contribute to the present day prevalence of hypothyroidism in young adult dogs.

Reference: Castillo, Rodriguez, Lalia, Pisarev. Int J Appl Vet Med 1(1): 1-8, 2003.

Healthy greyhounds (3 male, 3 female) were studied to assess pharmacokinetics and pharmacodynamics of morphine and the effects of ketoconazole on these parameters.

Morphine sulfate (0.5 mg/kg. IV) was administered to greyhounds prior to and after 5 days of ketoconazole (12.7 ± 0.6 mg/kg, PO) treatment. Plasma samples were obtained from blood samples collected at predetermined time points for measurement of morphine and ketoconazole concentrations by mass spectrometry. Pharmacokinetics of morphine were estimated by use of computer software.

Pharmacodynamic effects of morphine in greyhounds were similar to those of other studies in dogs and were similar between treatment groups. Morphine was rapidly eliminated with a half-life of 1.28 hours and a plasma clearance of 32.55 mL/min/kg. The volume of distribution was 3.6 L/kg. No significant differences in the pharmacokinetics of morphine were found after treatment with ketoconazole. However, plasma concentrations of ketoconazole were high and persisted longer than expected in greyhounds.

Conclusions and Clinical Relevance Ketoconazole had no significant effect on morphine pharmacokinetics, and the pharmacodynamics were similar between treatment groups. Plasma concentrations of ketoconazole were higher than expected and persisted longer than expected in greyhounds.

Reference: Kukanich and Borum. AJVR 69:664–669; 2008.