Thyroid Function Tests

Thyroid-stimulating hormone (TSH) is the preferred initial serum test for suspected thyroid dysfunction and for screening. It reflects the feedback loop between the pituitary gland and circulating thyroid hormone. The pituitary functions like a thermostat sensor. When circulating thyroid hormone is too low, it releases more TSH to stimulate the thyroid. When thyroid hormone is excessive, TSH production is suppressed. This inverse relationship makes TSH a sensitive first signal of primary thyroid disease. For most nonpregnant adults, the TSH reference interval is approximately 0.4–4.0 mIU/L. Always use the interval reported by the laboratory because assays and population ranges differ. A result outside the range is a prompt for clinical evaluation and, when indicated, targeted hormone testing; it is not a diagnosis by itself. TSH becomes less reliable when the pituitary or hypothalamus is the problem. In central hypothyroidism, the thyroid may receive too little stimulation even though it is capable of producing hormone. The TSH may be low, normal, or only mildly elevated despite a low free T4. In that situation, a “normal” TSH is not reassuring.