Thyroid hormone assays

FT4 assays based on direct equilibrium dialysis or ultrafiltration measure free hormone without the need for total hormone measurements. These methods are unaffected by either variations in serum binding proteins or thyroid hormone autoantibodies. However, IV heparin administration can cause spurious elevations m FT4 determined by these techniques as a consequence of in vitro generation of free fatty acids. Mean values obtained in euthyroid healthy subjects are reported to be slightly higher when using ultrafiltration methods than when using equilibrium dialysis. Analytical performance goals have been recommended for free thyroid hormone assays.When an FT4 assay... 

Work-up for goiter includes a thorough history and physical examination thyroid hormone assays, e.g., TSH, T4, triiodothyronine (T3) and an ultrasound or CT scan. 

Chemical Assay. In view of the similarity of their chemical and physical properties (see Table 1) (29), the main problem in the chemical analysis of the thyroid hormones is their separation. A USP procedure gives the details of a paper chromatographic separation in which T is examined for contamination by T and 3,5-diiodothyroiiine (30). Other systems are also employed (29). 

Gutleb, A.C., Schriks, M., and Mossink, L. et al. (2007). A synchronized amphibian metamorphosis assay as an improved tool to detect thyroid hormone disturbance by endocrine disrupters and apolar sediment extracts. Chemosphere 70, 93-100. 

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). 

Thyroid Hormone Receptor (TR) can bind to the LTR in vivo independently of its ligand and regulates promoter activity. ChIP assays with anti-acetylated-histone antibodies revealed that unliganded TR reduce the local histone acetylation levels at the HIV-1 LTR, while thyroid hormone treatment reverses this induction (Hsia and Shi, 2002). Accordingly, unliganded TR recruits co repressors and at least one HDAC (Hsia and Shi, 2002). 

OECD (2006) Detailed review paper on thyroid hormone dismption assays. OECD series on testing and assessment. OECD, Paris... 

OECD. 2006f. Detailed review paper on thyroid hormone disruption assays. OECD Series on Testing and Assessment No. 57. Environment Directorate, Joint Meeting of the Chemicals Committee and the Working Party on Chemicals, Pesticides and Biotechnology. ENV/JM/MONO(2006)24. Paris OECD. 

Management of hypothyroidism consists of identifying the underlying cause and then providing thyroid hormone replacement to normalize thyroid sta-ms. The goal of treatment is to reduce semm TSH levels to normal, which for most assays is roughly between 0.5 and 3 mU/1. Oversuppression of TSH levels is probably not advisable, as overtreatment may predispose to cardiac arrhythmias (particularly atrial fibrillation), and may have subtle effects on bone mineral density. 

The total thyroid hormone content of thyroid glands and the ratio of T3 to T4 vary somewhat from one species to another. Thyroid USP preparations are therefore standardized on the basis of their iodine content. Much of the iodine in these preparations is in the metabolically inactive form of iodotyrosines. Thus, a given preparation may satisfy the USP iodine assay requirements and yet contain low amounts of T4 and T3. Thyrar (a beef extract) and Armour Thyroid tablets (a pork extract) are evaluated by additional biological assays to ensure consistent potency from one batch to another. 

Freitas, J., Cano, P, Craig-Veit, C., Goodson, M.L., Furlowe, Murk, A.J. (2009). Detection of thyroid hormone receptor disrupters by a novel stable in vitro reporter gene assay. Submitted. 

Polybrominated Diphenyl Ethers. Results of in vitro estrogen receptor and thyroid hormone transport protein binding assays and in vivo studies of thyroid hormone homeostasis indicate that PBDEs have the potential to disrupt normal endocrine function. 

The same 17 PBDE congeners and three hydroxylated PBDEs were also tested for possible interaction with T4 binding to human TTR, a plasma transport protein of thyroid hormones, in an in vitro competitive binding assay (Meerts et al. 1998, 2000). None of the pure congeners competed with T4 for binding to... 

Physiologic Function Testing, An example of this application is the assay of thyroid hormone levels in (lie blood winch, in turn, can aid in the assessment of thyroid function. The radioactive iodine uptake test, which involves the administration of a dose of l31l (iodine-131) to the patient, is also a valuable procedure in assessing thyroid function. At present, the technique is best reserved for problem cases rather than used as a primary screening test. The main disadvantage of this test is the effect of the dietary intake of iodine, which reacts in various ways in different individuals. 

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. 

Visser TJ, Kaptein E, van Toor H, et al. Glucuronidation of thyroid hormone in rat liver effects of in vivo treatment with microsomal enzyme inducers and in vitro assay conditions. Endocrinology 1993 133 2177-2186. 

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