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Hypothyroidism is insufficient thyroid-hormone action resulting from inadequate hormone availability in the usual clinical setting. Dysfunction can arise in the thyroid or in its upstream control. Measuring thyroid-stimulating hormone and thyroid hormones helps distinguish those pathways.^[ref]

Thyroid-stimulating hormone is a pituitary hormone regulating thyroid activity. Its concentration is interpreted together with thyroid hormone measurements and the control pathway involved. Reduced thyroid output and impaired upstream stimulation can produce different combinations of measured values.^[ref]