Thyroid- Stimulating Hormone T he mainstay of the diagnosis of thyrotoxicosis is measurement of serum TSH and thyroid hormones. In fact, many laboratories measure only TSH in the initial assessment and measurement of thyroid hormones is automatically added only if TSH is abnormal (‘TSH reflex’). Indeed, TSH concentration is inversely log- linearly proportional to fT4 level. The current immunoassays are very sensitive and can measure TSH levels well below the normal range, with a functional sensitivity of less than 0.02 mU/ L. Since pituitary TSH secretion is tightly downregulated by thyroid hormone level, TSH is undetectable in most cases of thyrotoxicosis. The only re markable exception are TSH- secreting adenomas, in which a high or inappropriately normal TSH level is found in spite of overt thyrotoxicosis. Because of the sensitivity of the assay, low (less than 0.4 mU/ L) but detectable TSH levels can be found. These levels are encountered in subclinical thyrotoxicosis and in other conditions, such as non- thyroidal illnesses, endogenous or exogenous corticosteroid excess. TSH is a heterogeneous molecule and different TSH isoforms circulate in the blood and are present in pituitary extracts used for assay standardization. Although current methods have eliminated cross- reactivity with other glycoprotein hormones, they may detect different epitopes of abnormal TSH isoforms, secreted by some euthyroid individuals and some patients with pituitary diseases. Rarely, the presence in the serum of antimouse immunoglobulin antibodies may interfere in the TSH assay, causing falsely elevated TSH levels.

Thyroid Hormone

 Measurement of serum thyroid hormone levels is mandatory in all patients with suspected thyrotoxicosis, for a proper evaluation of a low TSH level and for an estimation of the severity of the dis ease. The active form of the hormones in serum is the very small amount of freely circulating T4 (free T4– fT4) and T3 (free T3– fT4), which can enter cells, interacting with the specific receptors. Total T4 (tT4) and total T3 (tT3) can be easily and inexpensively measured by radioimmunoassay, but their levels are influenced by the levels of binding protein, which vary in healthy subjects and may change in several conditions. Thus, total thyroid hormone levels may not parallel those of free thyroid hormones, and their measurement is nowadays considered less useful in the evaluation of thyrotoxicosis. Free thyroid hormone levels measurements, although not completely exempt from flaws, are therefore more satisfactory, since they provide a more accurate measurement of the active hormone.

In iodine- sufficient countries a single measurement of fT4 is sufficient to confirm or reject the suspicion of thyrotoxicosis and, after TSH measurement, this is the test most often used in North America for thyroid function screening. In contrast, in iodine- deficient countries, a significant proportion of hyperthyroid patients (up to 12%) may have elevated fT3 and normal fT4 levels (T3- toxicosis). Conversely, fT4 can be falsely elevated in conditions causing reduced peripheral conversion of T4 to T3. In our practice, when thyrotoxicosis is suspected, we initially assess both fT4 and fT3 levels along with TSH with little additional expense in order to obtain a complete assessment of the thyroid function status.

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مواضيع ذات صلة

Laboratory Findings of Beta-Thalassemia Syndromes

Complement Fixation Assays

Flocculation Tests

Particle Agglutination Tests

Direct Whole Pathogen Agglutination Assays

Separating IgM From IgG For Serologic Testing

Serodiagnosis of Infectious Diseases

TSH- Receptor Antibodies

Thyroglobulin and Thyroid Peroxidase Antibodies

Interpretation of Serologic Tests

Features of the Humoral Immune Response Useful in Diagnostic Testing

Enzyme Immunoassays