Thyrotropic feedback control
From Ask Dr Wiki
The pituitary gland secretes the glandotropic hormone thyrotropin (TSH) that stimulates secretion of thyroxine (T4) and triiodothyronine (T3) in the thyroid gland. Conversely, thyroid hormones inhibit secretion of TSH in the form of degenerative feedback so that the system achieves equilibrium levels of the involved hormones. Furthermore, TSH-incretion depends from the level of the releasing hormone TRH that is formed in the hypothalamus. TRH thus acts in a way similar to a setpoint of the feedback control.
This main feedback interaction is complemented by additional "plugged in" feedback loops, e.g. an ultra-short-feedback control where TSH inhibits its own secretion, a long-feedback loop connecting thyroid hormones with TRH secretion and feedback loops for plasma protein binding of T4 and T3.
Functional states of pituitary-thyroid feedback control
- Euthyroidism (normal thyroid function)
- Hypothyroidism (thyroid hypofunction)
- Primary hypothyroidism (feedback control disconnected in the thyroid gland, e. g. by low secretory capacity after thyroid surgery or in autoimmune thyroiditis)
- Secondary hypothyroidism (feedback control disconnected in the pituitary gland, e.g. in the case of anterior pituitary insufficiency)
- Tertiary hypothyroidism (missing set point by lack of TRH, e.g. in the case of hypothalamic damage, of Pickardt's syndrome or non-thyroidal illness syndrome)
- Thyrotoxicosis (excess supply of the organism with thyroid hormones, e.g. by hyperthyroidism or drugs)
- Thyroid hormone resistance (feedback control disconnected at the site of thyroid hormone receptors in the pituitary and in periphere tissues)
The feedback control's function can be assessed in most cases by determining the following hormones:
In special cases the following parameters are required: