Thyrotropin alfa (recombinant human thyroid stimulating hormone) is a heterodimeric glycoprotein produced by recombinant DNA technology. It is comprised of two non-covalently linked subunits. The cDNAs encode for an alpha subunit of 92 amino acid residues containing two N-linked glycosylation sites, and a beta subunit of 118 residues containing one N-linked glycosylation site. It has comparable biochemical properties to natural human Thyroid Stimulating Hormone (TSH). Binding of thyrotropin alfa to TSH receptors on thyroid epithelial cells stimulates iodine uptake and organification, and synthesis and release of thyroglobulin, triiodothyronine (T3) and thyroxine (T4).
In patients with well-differentiated thyroid cancer, a near total or total thyroidectomy is performed. For optimal diagnosis of thyroid remnants or cancer via either radioiodine imaging or thyroglobulin testing and for radioiodine therapy of thyroid remnants, a high serum level of TSH is needed to stimulate either radioiodine uptake and/or thyroglobulin release. The standard approach to achieve elevated TSH levels has been to withdraw patients from thyroid hormone suppression therapy (THST), which usually causes patients to experience the signs and symptoms of hypothyroidism. With the use of thyrotropin alfa, the TSH stimulation necessary for radioiodine uptake and thyroglobulin release is achieved while patients are maintained euthyroid on THST, thus avoiding the morbidity associated with hypothyroidism.
The pharmacokinetics of thyrotropin alfa were studied in patients with well-differentiated thyroid cancer following a single 0.9 mg intramuscular injection. After injection, the mean peak (Cmax) level obtained was 116 ± 38 mU/l and occurred approximately 13 ± 8 hours after administration. The elimination half-life was 22 ± 9 hours. The major elimination route of thyrotropin alfa is believed to be renal and to a lesser extent hepatic.
Non-clinical data are limited, but reveal no special hazard for humans from use of thyrotropin alfa.
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