T4 is converted into T3 at the cellular level in virtually all organs. Intracellular T3 levels regulate pituitary secretion and blood levels of TSH, as well as the effects of thyroid hormone in multiple organs. Therefore, in most people a normal TSH indicates either normal endogenous thyroid function or an adequate T4 replacement dose. TSH only becomes unreliable in patients with suspected or known pituitary or hypothalamic disease when TSH cannot respond physiologically to altered levels of T4 or T3. Patients should have access to additional testing, as required.
Sources:
Garber JR, et al. Clinical practice guidelines for hypothyroidism in adults: cosponsored by the American Association of Clinical Endocrinologists and the American Thyroid Association. Endocr Pract. 2012 Nov-Dec;18(6):988-1028. PMID: 23246686.
Related Resources:
CSEM Review and Response: Testing and Management of Primary Hypothyroidism
Toolkit: Give the Test a Rest: A toolkit for decreasing unnecessary emergency department laboratory testing