In canine hypothyroidism, which laboratory finding supports primary hypothyroidism?

• A. Elevated T4 with suppressed TSH
• B. Normal T4 with elevated TSH
• C. Low T4 with elevated TSH
• D. Low T4 with normal TSH

Answer: (C)

In primary hypothyroidism, the thyroid gland itself is failing to produce enough hormone. When T4 levels drop, the pituitary responds by releasing more TSH in an attempt to stimulate the thyroid. So the pattern you see is low T4 with an elevated TSH. That combination shows the problem lies in the thyroid gland, not in the pituitary or hypothalamus, which is why it’s the best indicator of primary hypothyroidism in dogs.

If T4 were high or normal with a suppressed or normal TSH, that would point away from hypothyroidism being the primary issue. If T4 is low but TSH is not elevated, that could suggest secondary hypothyroidism or other non-thyroid illness effects rather than a primary thyroid failure.