Hypothalamic–pituitary–thyroid axis

The hypothalamic–pituitary–thyroid axis (HPT axis for short, a.k.a. thyroid homeostasis or thyrotropic feedback control) is part of the neuroendocrine system responsible for the regulation of metabolism and also responds to stress.

As its name suggests, it depends upon the hypothalamus, the pituitary gland, and the thyroid gland.

The hypothalamus senses low circulating levels of thyroid hormone (Triiodothyronine (T3) and Thyroxine (T4)) and responds by releasing thyrotropin-releasing hormone (TRH). The TRH stimulates the anterior pituitary to produce thyroid-stimulating hormone (TSH). The TSH, in turn, stimulates the thyroid to produce thyroid hormone until levels in the blood return to normal. Thyroid hormone exerts negative feedback control over the hypothalamus as well as anterior pituitary, thus controlling the release of both TRH from hypothalamus and TSH from anterior pituitary gland.^[2]

The HPA, HPG, and HPT axes are three pathways in which the hypothalamus and pituitary direct neuroendocrine function.

: Normal thyroid function

Euthyroidism

Hypothyroidism

autoimmune thyroiditis

Hyperthyroidism

Graves' disease

: Over-supply with thyroid hormones, e.g. by overdosed exogenously levothyroxine supplementation.

Thyrotoxicosis

and high-T3 syndrome: Consequences of step-up hypodeiodination, e.g. in critical illness as an example for type 1 allostasis,^[20] or hyperdeiodination, as in type 2 allostasis, including posttraumatic stress disorder.^[12]

Low-T3 syndrome

: Feedback loop interrupted on the level of pituitary thyroid hormone receptors.

Resistance to thyroid hormone

(thyrotropin, thyroid stimulating hormone)

TSH

Free T4

Free T3

Standard procedures cover the determination of serum levels of the following hormones:

For special conditions the following assays and procedures may be required:

Thyroid function tests

Hypothalamic–pituitary–adrenal axis

Hypothalamic–pituitary–gonadal axis

Hypothalamic–neurohypophyseal system

a free computer simulation for thyroid homeostasis in humans

SimThyr

Dietrich J W, Tesche A, Pickardt C R, Mitzdorf U (2004). "Thyrotropic Feedback Control: Evidence for an Additional Ultrashort Feedback Loop from Fractal Analysis". . 35 (4): 315–331. doi:10.1080/01969720490443354. S2CID 13421388.{{cite journal}}: CS1 maint: multiple names: authors list (link)

Cybernetics and Systems

Gauna C, van den Berghe G H, van der Lely A J (2005). "Pituitary Function During Severe and Life-threatening Illnesses". . 8 (3–4): 213–217. doi:10.1007/s11102-006-6043-3. PMID 16508715. S2CID 22305001.{{cite journal}}: CS1 maint: multiple names: authors list (link)

Pituitary

Dietrich, Johannes W.; Midgley, John E. M.; Hoermann, Rudolf (2018). Homeostasis and allostasis of thyroid function. Lausanne: Frontiers Media SA. 9782889455706.