Endocrine system is the second key regulator of various body system functions after nervous system(2013,AbdAlla). One of the largest endocrine glands in the body is The Thyroid gland It is positioned on the neck (2014,Peepre et al) just below the Larynx and has two lobes with one on either side of the trachea. The thyroid gland is genetically programmed to be the metabolic regulator in all vertebrates because It is involved in the production of two amino acid-iodine bound hormones known as 3-5-3’ triiodothyronine (T3) and 3-5-3’-5’ tetraiodothyronine (T4, thyroxine) The thyroid gland secretes the hormone thyroxine (T4) along with small amounts of triiodothyronine (T3). The most of the T3 in the circulation (about 80%) is formed from the conversion of T4 to T3 by the deiodinase enzyme. Most of this transformation takes place in the kidney and liver. T3 is considered the physiologically active hormone; in this sense T4 can be thought as a prohormone(2011,Russell and Joffe) .
The thyroid also produces and releases the hormone calcitonin (thyrocalcitonin) which contributes to the regulation of blood calcium levels. calcitonin reduced the concentration of calcium in the blood. Majority of the calcium removed from the blood is stored in the bones.
The Production of T3 and T4 are regulated by thyroid stimulating hormone (TSH), which produced by the pituitary gland. Higher levels of TSH lead to higher rates of hormone production and secretion from the thyroid. TSH in turn is regulated by another hormone secreted from the hypothalamus, thyrotropin-releasing hormone (TRH). TSH levels are also regulated in a negative feedback manner by the levels of circulating thyroid hormone, when T3 and T4 levels are too low TSH Production is increased (2000,Hulbert) . These hormones increase the metabolic activity of the body cells and released throughout the body to direct the body metabolism. They stimulate all body cells to work at a better metabolic rate. Without these hormones the body s cells would not be able to regulate the speed at which they performed chemical actions. Their release will be increased under certain situations such as cold temperatures when a higher metabolism is needed to generate heat(1975,Balsam and Sexton).
Anabolic–androgenic steroids (AAS) are a class of compounds that include any drug or hormonal substance, pharmacologically and chemically related to testosterone that stimulates the growth or manufacturing of bone and muscle(anabolic effect) (2008,Salas-Ramirez et al) These substances are frequently misused by athletes, bodybuilders and youths in order to increase muscle mass or enhance physical endurance (2004,Bahrke and Yesalis; 2005,Thiblin and Petersson) Of AAS, nandrolone, 19-nortestosterone (C18H26O2), has a stronger anabolic capacity (about 5 times higher) than testosterone (2006,Chrousos). Nandrolone is either chemically synthesized or naturally found in some mammals, including human and farm animals. Thyroid gland is one of the non-classical target organs for sex steroids (2006,Bermudez et al) . Thus Administration of sex hormones can interfere substantially with the hypothalamic–pituitary–thyroid (HPT) axis (2006,Bisschop et al,). Androgen administration in women decreases T4 substitution in patients with primary hypothyroidism (1994,Arafah).
Thyroid hormones regulate oxidative metabolism and thus play an important role in free radical production. Hence variations of thyroid hormone levels can be one of the main physiological modulators of in vivo cellular oxidative stress due to their known effects on mitochondrial respiration (1999,Guerrero et al.). These conditions determine a higher consumption of cellular antioxidants (1986,Sies; 1988 Fernandez et al.; 1998, Huh et al.) and inactivation of antioxidant enzymes (1988,Fernandez et al.), thus inducing oxidative stress (1986,Sies) with the concomitant increase in lipid peroxidation and protein oxidation (2011,Pasupathi and Latha)., One of the major effects of thyroid hormones is to increase mitochondrial respiration which results in increased generation of reactive oxygen species (ROS) (1978,Nishiki et al,).