What thyroid function test findings are indicative of primary hypothyroidism?

In primary hypothyroidism, the thyroid gland fails to produce sufficient thyroid hormones, which leads to specific hormone levels in the body. The pituitary gland responds to low levels of thyroid hormones (specifically FT4, or free thyroxine) by increasing the secretion of TSH (thyroid-stimulating hormone) in an attempt to stimulate the thyroid to produce more hormones.

When examining the thyroid function test findings indicative of primary hypothyroidism, an increased TSH level paired with decreased FT4 is the hallmark. This occurs because the body is trying to compensate for low thyroid hormone levels by signaling the thyroid gland to produce more, but due to the underlying dysfunction of the thyroid gland, it fails to do so effectively.

Thus, the combination of increased TSH and decreased FT4 serves as a clear indicator of primary hypothyroidism, confirming that the issue lies within the thyroid gland itself, rather than being a result of pituitary or hypothalamic problems.