Which laboratory findings would be expected in hypothyroidism?

In hypothyroidism, the expected laboratory findings include low levels of thyroxine (T4) and elevated levels of thyroid-stimulating hormone (TSH). This occurs because the thyroid gland is not producing sufficient amounts of T4, leading to a drop in the hormone's levels in the bloodstream. In response to low T4 levels, the pituitary gland secretes more TSH to stimulate the thyroid to produce more hormones.

This feedback loop is crucial in understanding thyroid function. The high TSH levels indicate that the pituitary gland is working hard to encourage the thyroid gland to produce more thyroid hormones, but since the thyroid is underactive or dysfunctional, T4 remains low.

In contrast, elevated T4 and TSH levels would suggest hyperthyroidism, where the thyroid is producing too much hormone; normal T3 and low T4 may indicate certain types of thyroid dysfunction, but they do not fit the classic signs of hypothyroidism; and low T4 with low T3 might suggest a different issue altogether, such as severe illness or a non-thyroidal illness. Thus, low T4 combined with high TSH is the hallmark laboratory finding in hypothyroidism.