In addition to low free T4, what other lab finding is essential for diagnosing hypothyroidism?

For diagnosing hypothyroidism, the essential lab finding in addition to low free T4 is high TSH. In this condition, the thyroid is underactive, leading to insufficient levels of thyroid hormones (like T4), which triggers the pituitary gland to produce more TSH (thyroid-stimulating hormone) in an attempt to stimulate the thyroid gland to produce more hormones. Therefore, in primary hypothyroidism, one would expect the TSH levels to be elevated due to this feedback mechanism.

While low free T4 is a primary indicator of hypothyroidism, the measurement of TSH is critical for confirming the diagnosis, as it not only indicates that the pituitary is responding to low levels of thyroid hormones but also helps differentiate primary hypothyroidism from secondary causes.

Low TSH would indicate an expected finding in conditions such as hyperthyroidism or could suggest central causes of hypothyroidism, and while normal T3 levels might be observed in certain subclinical forms, they are not critical for the diagnosis of overt hypothyroidism. High cortisol is unrelated to thyroid dysfunction and would not assist in making a diagnosis of hypothyroidism.