What is indicated by an older dog exhibiting a high total T4 and high TSH level?

In older dogs presenting with a high total T4 and a high TSH level, the findings are indicative of a thyroid condition known as idiopathic thyroiditis. In idiopathic thyroiditis, there is an immune-mediated destruction of the thyroid gland, which results in high levels of thyroid hormones (T4) being released into circulation. As the thyroid hormone levels rise, the pituitary gland senses this high thyroid hormone level and consequently increases the production of thyroid-stimulating hormone (TSH).

This combination of high T4 and high TSH suggests that the feedback mechanism is functioning, but the underlying cause is the ongoing destruction of thyroid tissue, which is characteristic of idiopathic thyroiditis. In contrast, primary hyperthyroidism is less common in dogs compared to cats and usually presents with a different profile where TSH would be low due to negative feedback. Secondary hyperthyroidism usually involves a pituitary tumor causing increased TSH and subsequently increased T4, which would not typically present with both elevated T4 and TSH simultaneously. Thyroid neoplasia more commonly leads to elevated T4 levels, but not necessarily elevated TSH, as the tumor often autonomously produces T4 without the regulatory pituitary influence.

Understanding these mechanisms is crucial for