Which laboratory results would most likely support a diagnosis of primary hypothyroidism?

A diagnosis of primary hypothyroidism is typically supported by laboratory findings that reflect the dysfunction of the thyroid gland itself, which results in low levels of thyroid hormones combined with elevated thyroid-stimulating hormone (TSH) levels. In primary hypothyroidism, the thyroid gland is unable to produce adequate amounts of thyroxine (T4) and possibly triiodothyronine (T3), leading to an accumulation of TSH as the pituitary gland attempts to stimulate the thyroid.

In this scenario, low levels of T4 indicate that the thyroid is not functioning properly, while the high levels of TSH confirm that the pituitary gland is responding to the low thyroid hormone levels by producing more TSH. This pattern of low T4 and high TSH is characteristic of primary hypothyroidism, distinguishing it from secondary causes of hypothyroidism, where TSH levels would be low due to inadequate stimulation from the pituitary.

Other laboratory results do not align with the presentation of primary hypothyroidism, as they suggest either normal thyroid function or inadequate levels of TSH, which would not support the diagnosis of primary thyroid failure.