Hypothalamic–pituitary–thyroid axis

The thyroid stimulating loop (synonyms: pituitary-thyroid loop, pituitary-thyroid axis, thyroid stimulating axis ) is a more circuitous creeping control loop, which is clamped between the pituitary gland and the thyroid gland. He regulates the concentration of thyroid hormones in the blood plasma.

Physiology

The pituitary gland, the control of hormone thyrotropin (TSH ), which in the thyroid stimulating the secretion of thyroxine (T4) and triiodothyronine (T3). Conversely inhibit thyroid hormone in a negative feedback ( Negative feedback) the production and secretion of TSH, so that usually an equilibrium level of the amount of thyroid hormones in the blood. The production and secretion of TSH also depends on the levels of thyrotropin releasing hormone, TRH and somatostatin, both of which are produced and released by the hypothalamus. The hypothalamus is the setpoint of the thyroid hormones in the blood and continuously measures the actual value. To adjust the set point of the thyroid hormones in the blood the actual value of the thyroid hormones in the blood, the hypothalamus can influence the production amount of TRH and thus the production amount of TSH and thyroid hormones.

Apart from this main loop, there are more turned on feedback loops, such as an ultra -short feedback of TSH to its own distribution ( the chunks - Wiersinga - Prummel - loop), a Long - feedback of thyroid hormones on TRH release and control circuits, which adjust the plasma protein binding of T4 and T3.

The TSH secretion is other than TRH, somatostatin and peripheral thyroid hormones is controlled by a variety of other afferent signals. This is probably one of the reasons that the relationship between FT4 and TSH differs from the previously propagated loglinear context.

The iodine uptake of the thyroid gland is not only the TSH level dependent, but is also subject to an iodine - dependent autoregulation. Gifts of large amounts of iodide (several hundred milligrams - the daily requirements of healthy individuals is given by the WHO with 200 micrograms) inhibit Iodidaufnahme, hormone synthesis and hormone release (Wolff- Chaikoff effect, by Louis Wolff, 1898-1972, American cardiologist and IL Chaikoff, American physiologist ). This effect that lasts only a few days, made you look earlier in the treatment of hyperthyroidism before thyroid surgery advantage ( " Plummern ", according to Henry Stanley Plummer).

The functions of the pituitary-thyroid loop

• Euthyroid ( normal thyroid function)
• Hypothyroidism ( underactive thyroid ) Primary hypothyroidism ( loop in the thyroid interrupted, eg due to lack of Inkretionsleistung after surgery or at Autoimmunthyreopathien )
• Secondary hypothyroidism ( loop in the pituitary interrupted, eg in the context of hypopituitarism )
• Tertiary hypothyroidism ( Setpoint entry is missing due to lack of TRH, eg in the context of damage to the hypothalamus, a Pickardt syndrome or euthyroid sick syndrome. )

• Primary hyperthyroidism ( Inappropriate secretion of thyroid hormones by thyroid problems, eg autonomies and in Graves' disease )
• Secondary hyperthyroidism (rare by TSH - producing tumors of the pituitary or thyroid hormone resistance )
• Tertiary hyperthyroidism ( conceivable by TRH overproduction in the hypothalamus or paraneoplastic TRH - formation, not yet observed)

Diagnostics

The function of the control loop can be determined in most cases by the determination of the following hormones:

• TSH ( thyroid stimulating hormone)
• Free T4
• Free T3

Only for special issues of the following thyroid function tests are required:

• Total T4
• Total T3
• TRH
• TBG
• TRH tests
• Secretion of the thyroid gland (GT )
• Sum activity of peripheral deiodinases ( GD)
• TSH index ( TSHI )