Also between the follicles are numerous blood capillaries , small lymphatic vessels , and connective tissue . The entire thyroid gland is encased in a thin outer layer of connective tissue. T 4 and T 3 play an important role in controlling body metabolism . Calcitonin acts in conjunction with parathyroid hormone to regulate calcium balance in the body. The function of the thyroid gland is controlled by both the pituitary gland and the hypothalamus , so thyroid disorders may be due not only to defects in the gland itself, but also to disruption of the hypothalamic-pituitary hormonal control system .
Goiter sometimes occurs without any accompanying abnormality of the thyroid function.