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Hypothyroidism is classified as either primary, secondary, or tertiary. Primary hypothyroidism is for when the cause is due to an abnormality of the thyroid gland, secondary hypothyroidism is when the cause is decreased thyroid-stimulating hormone levels, and tertiary hypothyroidism is when the cause is an inadequate amount of thyrotropin-releasing hormone being released.
The cause of this is not known. When a dog has lowered serum levels of thyroid hormones and endocrine alopecia it is harder to differentiate between hypothyroidism and Cushing's. Signs of polydipsia, polyuria, and polyphagia help to differentiate as these symptoms do not occur in hypothyroidism.
The diagnosis of hyperthyroidism is confirmed by blood tests that show a decreased thyroid-stimulating hormone (TSH) level and elevated T 4 and T 3 levels. TSH is a hormone made by the pituitary gland in the brain that tells the thyroid gland how much hormone to make. When there is too much thyroid hormone, the TSH will be low.
Certain medications can have the unintended side effect of affecting thyroid function. While some medications can lead to significant hypothyroidism or hyperthyroidism and those at risk will need to be carefully monitored, some medications may affect thyroid hormone lab tests without causing any symptoms or clinical changes, and may not require treatment.
The syndrome can present with variable symptoms, even between members of the same family harboring the same mutation. [1] Typically most or all tissues are resistant to thyroid hormone, so despite raised measures of serum thyroid hormone the individual may appear euthyroid (have no symptoms of over- or underactivity of the thyroid gland).
Stimulating antibodies to the TSH receptor mimic TSH and cause Graves' disease. In addition, hCG shows some cross-reactivity to the TSH receptor and therefore can stimulate production of thyroid hormones. In pregnancy, prolonged high concentrations of hCG can produce a transient condition termed gestational hyperthyroidism. [14]
Fasting in healthy, euthyroid people causes reduced T3 and elevated rT3, although TSH is usually unchanged. [3] [5] [10] Even moderate weight loss can lower T3. [2] This may be primarily via reduced levels of leptin (the satisfaction hormone). Low leptin levels can downregulate hypothalamic TRH neurons and cause a reduction in TSH.
Acetaminophen (paracetamol, Tylenol) can cause liver damage in dogs. The toxic dose is 150 mg/kg. [174] Ibuprofen * can cause gastrointestinal irritation, stomach ulcers, and kidney damage in dogs. [175] Naproxen (Aleve)* has a long half-life in dogs and can cause gastrointestinal irritation, anemia, melena (digested blood in feces), and vomiting.