Thyroid hormones measurement

Thyrotoxicosis factitia should be suspected in a thyrotoxic patient without evidence of increased hormone production, thyroidal inflammation, or ectopic thyroid tissue. The RAIU is low because thyroid gland function is suppressed by the exogenous thyroid hormone. Measurement of plasma thyroglobulin reveals the presence of very low levels. 

A. Bishnoi, et al., Effect of commonly prescribed nonsteroidal anti-inflammatory drugs on thyroid hormone measurements. Am. J. Med. 96 235-238, 1994. 

A number of medications have been shown to alter thyroid function and thyroid function tests.Few drugs are associated with the development of clinically significant thyroid disease (amiodarone may be an exception), but difficulty in the interpretation of thyroid function tests results when patients are placed on medications that affect thyroid function testing. In general, drugs do not interfere chemically with the assays for thyroid hormones or TSH. The medications most likely to affect TSH concentrations are glucocorticoids and dopamine (reduced TSH concentrations) and amiodarone (increased TSH concentrations). The most commonly encountered variations in thyroid hormone measurements induced by medications are reduced peripheral conversion of T, to T3 or altered binding of T4 and T3 to carrier proteins. Some of the medications that affect thyroid function tests are shown in Table 52-2. 

Numerous methods have been developed for assessing the concentrations of FT4 and FT3 in serum. These methods include direct assays that currently serve as reference methods and indirect assays that are more widely available for general laboratory use. The following section describes the principles of these methods and offers some guidelines for their use. The theoretical basis, analytical validity, and clinical utility of these methods have been discussed. Special reports from the Nomenclature Committee of the American Thyroid Association, the National Academy of Clinical Biochemistry, and the NCCLS also review some of the issues and concerns regarding free thyroid hormone measurements. 

Stockigt JR. Chapter 18 Serum thyrotropin and thyroid hormone measurements and assessment of thyroid hormone transport. In Braverman LE, Utiger RD, eds. Werner and Ingbar s the thyroid. 7th ed. Philadelphia Lippincott-Raven, 1996 377-96. 

Hegstad-Davies, R. L. 2006. A review of sample handling considerations for reproductive and thyroid hormone measurement in serum and plasma. Theriogenology 66 592-598. 

Free thyroid hormone measurement in the diagnosis of thyroid disease. In... 

Consequently, the workshop recommends specific monitoring of iodine intake of mothers and infants in Europe by periodic analysis of urinary iodine levels, and to the extent feasible, of serum TSH and thyroid hormone measures. The daily intake of iodine should be at least 200 pg in pregnant and lactating women and 90-120 pg in young infants. To reach these objectives, the mothers diet should be systematically supplemented with iodine whenever necessary, by vitamins/minerals tablets as prescribed by physicians. Breast milk is the best source of iodine for the infant, and exclusive breast feeding for 4-6 months should be encouraged. However, when circumstances require that infants receive formula, the iodine content of formula milk should be increased fiom the traditional recommendation of S pg/dl milk to 10 pg/dl for full term and 20 pg/dl for premature babies. 

O In most patients with thyroid hormone disorders, the measurement of a serum thyroid-stimulating hormone (TSH) level is adequate for the diagnosis of hypothyroidism and hyperthyroidism. The target TSH for most patients being treated for thyroid disorders should be the mean normal value of 1.4 milliunits/L or 1.4 microunits/mL (target range 0.5-2.5 milliunits/L or 0.5-2.5 microunits/mL). 

H18. Hay, I.D., Bayer, M. F., Kaplan, M. M., Klee, G. G., Larson, P. R., and Spencer, C. A., American Thyroid Association assessment of current free thyroid hormone and thyrotropin measurements and guidelines for future clinical assays. Clin. Cliem. 37,2002-2008 (1991). 

In thyrotoxic Graves disease, there is an increase in the overall hormone production rate with a disproportionate increase in T3 relative to T4 (Table 20-1). Saturation of thyroid-binding globulin is increased due to the elevated levels of serum T4 and T3, which is reflected in an elevated T3 resin uptake. As a result, the concentrations of free T4, free T3, and the free T4 and T3 indices are increased to an even greater extent than are the measured serum total T4 and T3 concentrations. The TSH level is undetectable due to negative feedback by elevated levels of thyroid hormone at the pituitary. In... 

The following therapeutic measures should be instituted promptly (1) suppression of thyroid hormone formation and secretion (2) antiadrenergic therapy (3) administration of corticosteroids and (4) treatment of associated complications or coexisting factors that may have precipitated the storm (Table 20-2). 

General supportive measures, including acetaminophen as an antipyretic (aspirin or other nonsteroidal antiinflammatory drugs may displace bound thyroid hormone), fluid and electrolyte replacement, sedatives, digoxin, antiarrhythmics, insulin, and antibiotics should be given as indicated. Plasmapheresis and peritoneal dialysis have been used to remove excess hormone in patients not responding to more conservative measures. 

In another factor related to the thyroid gland, wide variation has been observed with respect to the thyroid-stimulating hormone (TSH) of the pituitary. D Angelo8 and co-workers, using tadpoles as a tool, have studied the thyroid and TSH activity of the serums of normal and diseased individuals. Both limb growth (metamorphosis) induced by thyroid hormone and the development of thyroid tissue induced by TSH were measured. 

Increased TBG leading to increased circulating total thyroid hormone, as measured by PBI, T4 by column, or T4 by radioimmunoassay. Free T3 resin uptake is... 

Patients with secondary or tertiary hypothyroidism are also usually treated with thyroxine, but the serum TSH concentration is not a reliable guide to therapy. The efficacy of thyroid hormone replacement in these patients must be assessed clinically and by measurement of the serum T4 concentration. 

Levothyroxine sodium (Levothwid, Synthroid, Levoxine) is the sodium salt of the naturally occurring levorota-tory isomer of T4. It is the preparation of choice for maintenance of plasma T4 and T3 concentrations for thyroid hormone replacement therapy in hypothyroid patients. It is absorbed intact from the gastrointestinal tract, and its long half-life allows for convenient once-daily administration. Since much of the T4 is deiodi-nated to T3, it is usually unnecessary to use more expensive preparations containing bothX4 and Tj.The aim is to establish euthyroidism with measured serum concentrations of T4, T3, and TSH within the normal range. 

Treatment is thyroid hormone replacement. The goal of the therapy is to relieve the symptoms of hypothyroidism by normalizing the levels of circulating thyroid hormones. In addition to the amelioration of symptoms, the clinical effectiveness of the thyroid hormone replacement may be monitored by periodically measuring the serum TSH concentration. The lowest dose of thyroid hormone that is needed to normalize the serum TSH concentration is usually the appropriate dose. Most or all of the symptoms of hypothyroidism should improve with appropriate thyroid hormone replacement, but this may require weeks or months of therapy. 

Carr D, McLeod DT, Parry G, Thornes HM. Fine adjustment of thyroxine replacement dosage comparison of the thyrotrophin releasing hormone test using a sensitive thyr-otrophin assay with measurement of free thyroid hormones and clinical assessment. Clin Endocrinol (Oxf) 1988 28(3) 325-33. 

Thyroid hormones and their structural analogs showed lower DPPH-scavenging activity in comparison with butylated hydroxytoluene (BHT) as a standard compound. 3,5,3, 5 -tetraiodothyroacetic acid, 3,3,5 -triiodo-L-thyronine, and thyroxine showed the highest antioxidant activity measured by DPPH reduction, 3,5,3 5 -tetraiodothyroacetic acid having over 20% of the activity of BHT (05). 

Because the irreversible mental retardation caused by neonatal hypothyroidism can be prevented by early treatment with thyroid hormone, the importance of early diagnosis of this disease has been emphasized. The difficulty of its early diagnosis by clinical features alone and its relatively high incidence have prompted the organization of various mass screening programs that include measuring thyroxine (T4) (D5) or TSH (13, Mil) by radioimmunoassay. 

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