Thyroid Function Tests

Thyroid-stimulating hormone (TSH) is the preferred first test when thyroid dysfunction is suspected. It detects both underactive and overactive thyroid states because it reflects communication between the pituitary gland and the thyroid. The pituitary acts like a thermostat with a sensitive feedback loop. When circulating thyroid hormone falls, the pituitary releases more TSH to stimulate the thyroid. When thyroid hormone rises, TSH production falls. This inverse relationship means a small change in free thyroid hormone can produce a more noticeable change in TSH. For most non-pregnant adults, a TSH result is interpreted against the laboratory reference interval, commonly about 0.45–4.5 mIU/L. Some authorities use approximately 0.4–4.0 mIU/L. Use the interval printed on the actual laboratory report rather than memorizing one universal cutoff. An abnormal TSH does not diagnose a patient in isolation. It directs the next question: is free thyroxine (free T4) normal, low, or high? That combination distinguishes overt hormone deficiency or excess from subclinical disease and identifies patterns that need further assessment.