Diagnostic patterns for primary vs central hypothyroidism, common lab pitfalls, and a pragmatic evaluation pathway.
Hypothyroidism is typically classified as primary (thyroidal failure) or central (pituitary/hypothalamic dysfunction). In primary hypothyroidism, TSH is usually elevated and free T4 reduced. Autoimmune thyroiditis (Hashimoto disease) is the most common etiology in iodine-sufficient settings and is supported by anti-TPO antibodies, though antibody positivity alone does not confirm overt disease. Central hypothyroidism requires a different mindset: free T4 is low while TSH may be low, normal, or only mildly elevated, because the secreted TSH can be biologically inactive. Therefore, relying on TSH alone can miss central disease, especially in patients with pituitary lesions, prior cranial irradiation, or multiple pituitary hormone deficiencies.
Laboratory pitfalls are common. Biotin supplementation can artifactually lower TSH and elevate free T4 on some immunoassays. Acute severe illness may reduce T3 and transiently alter TSH, mimicking thyroid dysfunction. Medications such as glucocorticoids, dopamine agonists, and amiodarone can influence the axis. Pregnancy changes binding proteins and reference intervals. A careful medication/supplement history and, when indicated, repeating tests after stopping biotin or during clinical stability can prevent misclassification.
A pragmatic approach begins with TSH and free T4, followed by thyroid antibodies if primary hypothyroidism is suspected. If central disease is possible, assess pituitary function (morning cortisol, prolactin, gonadotropins) and consider pituitary imaging. Treatment decisions should be based on biochemical severity, symptoms, pregnancy status, and cardiovascular risk, with careful titration and follow-up testing.
Laboratory pitfalls are common. Biotin supplementation can artifactually lower TSH and elevate free T4 on some immunoassays. Acute severe illness may reduce T3 and transiently alter TSH, mimicking thyroid dysfunction. Medications such as glucocorticoids, dopamine agonists, and amiodarone can influence the axis. Pregnancy changes binding proteins and reference intervals. A careful medication/supplement history and, when indicated, repeating tests after stopping biotin or during clinical stability can prevent misclassification.
A pragmatic approach begins with TSH and free T4, followed by thyroid antibodies if primary hypothyroidism is suspected. If central disease is possible, assess pituitary function (morning cortisol, prolactin, gonadotropins) and consider pituitary imaging. Treatment decisions should be based on biochemical severity, symptoms, pregnancy status, and cardiovascular risk, with careful titration and follow-up testing.
Published February 10, 2026