Clinical utility and cost-effectiveness of sensitive thyrotropin assays in ambulatory and hospitalized patients

In either an ambulatory or a hospitalized patient setting, a normal serum sensitive thyrotropin (TSH) value is strongly suggestive of euthyroidism if the patient has intact hypothalamic-pituitary function and is not receiving drugs known to suppress pituitary TSH secretion. In stable ambulatory patients, an abnormal sensitive TSH value is strongly suggestive of clinical or subclinical thyroid hormone excess or deficiency, which can be confirmed by a free thyroxine (T4) index (FT4I) and evaluation for antimicrosomal antibody (AMA) as a marker of autoimmune thyroid disease. In a hospitalized patient, an abnormality in sensitive TSH or FT4I does not necessarily indicate a thyroid problem but may merely reflect a nonthyroidal illness or glucocorticoid or dopamine treatment. A thyrotropin releasing hormone (TRH) test may be needed to diagnose hyperthyroidism in a hospitalized patient with a basal sensitive TSH level of less than 0.1 microU/ml because a detectable TRH response contraindicates hyperthyroidism whereas hyperthyroid patients with nonthyroidal illness have the expected absent response. In a hospitalized patient, hypothyroidism must be diagnosed on the basis of both a high TSH level and a low FT4I because an isolated high TSH value may merely reflect the recovery phase of a nonthyroidal illness. No clinical urgency exists for establishing a diagnosis of subclinical hypothyroidism in a hospitalized patient; definitive determination of thyroid status can be deferred until recovery and discharge.

The application of an immunoradiometric assay of plasma thyrotropin (TSH-IRMA) in molar pregnancy

Plasma thyrotropin was measured by immunoradiometric assay (TSH-IRMA, "Sucrosep", Boots Celltech) in three patients with molar pregnancy. Two patients were hyperthyroid (increased plasma concentrations of FT4, FT4 index, and FT3 index), one patient was euthyroid (normal free thyroid hormone concentrations). TSH-IRMA values were below the detection limit (less than 0.1 mU/L) in the two hyperthyroid patients; the euthyroid patient had a TSH-IRMA value of 0.8 mU/L, within the reference range of normal subjects (0.5-3.3 mU/L). In vitro experiments demonstrated interference of the grossly elevated hCG levels in the TSH-IRMA, resulting in lower values. The observed interference was independent of ambient TSH concentrations, but related to hCG levels. We conclude that despite interference of hCG in TSH-IRMA the clinical usefulness of TSH-IRMA as a first-line test of thyroid function appears to be maintained in molar pregnancy.

Clinical value of a sensitive immunoradiometric assay for TSH

We have investigated a sensitive immunoradiometric assay for serum thyroid-stimulating hormone (TSH) in 45 thyrotoxic patients not on treatment, 464 euthyroid patients and 111 hypothyroid patients on replacement therapy. Forty-three thyrotoxic and seven euthyroid patients were found to have an undetectable TSH (less than 0.2 mIU/L). Previous work has shown a very clear separation of thyrotoxic and euthyroid patients using sensitive TSH assays. However, our extended study here has revealed that a significant number of euthyroid patients with undetectable TSH (1.5% in our study) are likely to be found if TSH becomes the initial test for thyroid function. Thirty of the hypothyroid patients on thyroxine were found to have undetectable TSH, but only one showed clinical signs of thyrotoxicosis. Most of the patients, although having raised serum free T4, had serum free T3 levels within the euthyroid range or just slightly elevated.