Both versions are then enveloped in a relatively large glycoprotein complex called thyroglobulin and stored in the thyroid gland. To be released into the bloodstream for circulation throughout the body, the hormones are separated from thyroglobulin and bound to a much smaller globulin thyroxin-binding globulin or albumin. However, only % of thyroid hormone is "free" to be biologically active. Thyroid's action in the cell is to increase the biosynthesis of enzymes, resulting in heat production, oxygen consumption, and elevated metabolic rate. Thyroid stimulates the oxidation of fatty acids, and reduces cholesterol by oxidizing it into bile acids. Thyroid also stimulates enzymes for protein synthesis and, when present in excessive amounts, can catabolize (destroy) muscle protein. Estrogen causes food calories to be stored as fat. Thyroid hormone causes fat calories to be turned into usable energy. Thyroid hormone and estrogen have opposing actions. Estrogen inhibits thyroid action in the cells, interfering with the binding of thyroid to its receptor. Both hormones have phenol rings at a corner of their molecule. The respiratory enzymes of cells are thyroid-dependent. When thyroid function is low, cellular oxygen is low (cellular hypoxia). Thus, estrogen-induced thyroid interference contributes to less-than-optimal brain function. Excess estrogen may compete with thyroid hormone at the site of its receptor. In so doing, the thyroid hormone may never complete its mission, creating hypothyroid symptoms despite normal serum levels of thyroid hormone. Progesterone, on the other hand, increases the sensitivity of estrogen receptors for estrogen and yet, at the proper level, inhibits many of estrogen's side effects. GABA (gamma-aminobutyric acid) is an amino acid that acts as a neurotransmitter-inhibitor and tends to have a calming effect. When estrogen interferes with thyroid production and slows the metabolism of brain cells, it indirectly decreases GABA production and increases brain cell excitability, a factor in epilepsy.