Objectives:
To evaluate the reliability of normal Thyroid Stimulating Hormone (TSH) as a thyroid function test and assess the effect of Adrenocorticotropic Hormone (ACTH) on serum TSH concentration.
Design and methods:
Patients presenting to the National Institutes of Health Department of Endocrinology outpatient clinic with symptoms consistent with hypothyroidism were identified. Thyroid hormone concentrations were measured by liquid chromatography/tandem mass spectrometry and immunoassay. Patients with normal TSH concentrations were assessed for both clinical and biochemical hypothyroidism.We evaluated the effect of ACTH stimulation (performed on patients for assessment of adrenal function) on TSH concentration.
Results:
Patients with symptoms consistent with hypothyroidism but with normal TSH values in the range of 1-4 IU/mL and normal free T4 (FT4) values by immunoassay measurements were confirmed to be biochemically hypothyroid following measurements of thyroid hormones by mass spectrometry. We present case studies of two patients, a 76-year-old male and a 58-year-old female. Improvement in the male patient's hypothyroid symptoms, including afternoon fatigue, constipation, alopecia, dry skin and high cholesterol, was documented after initiating thyroid hormone replacement.ACTH stimulation resulted in an average decrease of 17% in TSH between time 0 and 60 minutes post stimulation.
Conclusion:
Although measurement of TSH is a convenient screen for thyroid function, it is influenced by many factors which may affect its overall reliability. We believe thyroid function should be assessed by more than a single test. We recommend measurement of thyroid hormone concentrations by mass spectrometry if the patient's clinical presentation is discordant with their TSH levels.