"Currently, measurement of the serum TSH concentration is the most reliable indicator of thyroid status at the tissue level. Studies of mild (subclinical) thyroid hormone excess or deficiency (abnormal TSH/normal range FT4 and FT3) find abnormalities in markers of thyroid hormone action in a variety of tissues (heart, brain, bone, liver and kidney). These abnormalities typically reverse when treatment to normalize serum TSH is initiated" [1]

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[1] https://www.aacc.org/-/media/Practice-Guidelines/Thyroid-Dis...