Main Influences on Thyroid Function Tests
Physiologic Influences
Certain breeds such as sight hounds and performance dogs such as racing sled dogs have TT4 serum concentrations physiologically lower (up to half) than established laboratory reference intervals (Lee et al.
2004; van Geffen et al. 2006; Pinilla et al. 2009). This can complicate the diagnosis of hypothyroidism, especially if only serum TT4 is measured. Often free T4 (after equilibrium dialysis) is less different compared to other breeds. Serum TT4 concentrations tend to mildly decrease with age and intense exercise in sled dogs (Lee et al. 2004).Table 11.1 Summary of the most common clinical signs observed in dogs with hypothyroidism.
| Frequent | Less frequent | Doubtful relationship |
| Lethargy/weakness Obesity Alopecia/hypotrichosis Seborrhea Pyoderma or recurrent otitis | Neuropathy, vestibular syndrome Female infertility Myxoedema Lipid keratopathy | Male infertility Larynx paralysis Mega-esophagus Behavioral changes (aggressive) |
Table 11.2 Advantages and limitations of the most commonly used thyroid tests.
| Test | Advantages | Disadvantages |
| TT4 | Readily available, not expensive Normal values often allow “exclusion” of hypothyroidism | J with systemic disease (euthyroid sick syndrome) J after administration of certain drugs A J T4 alone does not allow a reliable diagnosis of hypothyroidism (low specificity) |
| TSH | Readily available, not expensive | 1/4 of hypothyroid dogs have TSH values within the reference interval (low sensitivity) Always use in combination with T4 |
| FT4 | Is less influenced by systemic disease or drug administration than TT4 | The only reliable method includes equilibrium dialysis Not readily available in all countries |
| Anti-thyroglobulin Ab | Testing for thyroid autoimmunity | Not routinely available in all countries Does not reflect thyroid function Positive in approximately 50% of hypothyroid dogs |
| Scintigraphy (99mTc04-) | Reliable, considered as a gold standard | Limited availability Use of radio nuclides Sometimes need for sedation |
| Thyroid ultrasonography | Theoretically interesting | Very operator- and machine dependent |
| TSH stimulation test | Reliable, considered as a gold standard (use rhTSH) | rhTSH[*] is expensive (less if aliquoted) 6 hours lasting test Anaphylactic reactions were described with bovine TSH (not yet with rhTSH) |
Iatrogenic Influence: Effect of Medication
The influence of certain drugs on thyroid function tests has long been underestimated in dogs (Daminet and Ferguson 2003). Table 11.3 gives a summary of the influence of drugs on canine thyroid function tests.
Measurement of serum TT4 or TSH should preferably be performed when the patient is not receiving any medication except if it has been demonstrated that no effect is to be expected as with imepitoin (Bossens et al. 2016).Table 11.3 Effects of drugs on canine thyroid results.
* Sulfonamide-induced hypothyroid crisis has been reported.
Source: Daminet 2003. American College of Veterinary Internal Medicine.
Pathologic Influences: Euthyroid
Sick Syndrome
The presence of a systemic non-thyroidal disease, such as diabetes mellitus, liver disease, hyperadrenocorticism and renal- or heart failure, is a frequent cause for decreased thyroid hormone concentrations. This phenomenon is referred to as the “euthyroid sick syndrome” (Scott-Moncrieff 2010). These changes probably reflect a physiological adaptation of the organism leading to a decrease in tissue energy requirements. The administration of synthetic thyroid hormones to these patients is not recommended.