Where is it produced and how is it transported?
Thyroliberin is synthesized by neurons of the parvocellular nuclei located in the medial region of the hypothalamus. These cells have dual properties: as neurons, they generate electrical impulses, and as endocrine cells, they produce a specific neurosecretion.
The hormone is transported via the hypophyseal portal system:
- The axons of the neurons project to the median eminence.
- The neurosecretion is released into the hypophyseal portal system.
- The portal system delivers the hormone to the cells of the anterior pituitary gland (adenohypophysis).
What are the targets of thyrotropin-releasing hormone?
The primary target of thyrotropin-releasing hormone is the adenohypophysis. It has a cross-effect and can influence several cell types of the adenohypophysis.
- Thyrotrophs: thyrotropin-releasing hormone stimulates the release of thyroid-stimulating hormone (TSH). TSH acts on the thyroid gland, stimulating the synthesis and secretion of iodothyronines — thyroxine ($T_4$) and triiodothyronine ($T_3$).
- Lactotrophs: thyrotropin-releasing hormone promotes an increase in prolactin secretion.
- Somatotropic cells: thyrotropin-releasing hormone stimulates somatotropic cells.
How is production regulated (feedback)?
The function of the hypothalamic-pituitary-thyroid axis operates via negative feedback mechanisms.
- Stimulation: in case of $T_3$ and $T_4$ deficiency, the secretion of thyrotropin-releasing hormone increases, which is accompanied by the stimulation of TSH secretion.
- Inhibition: high concentrations of iodothyronines ($T_3$ and $T_4$) inhibit TRH secretion at the hypothalamic level and TSH secretion at the pituitary level.
Clinical significance
Assessing thyrotropin-releasing hormone function is important for understanding the pathogenesis of endocrine diseases:
- Secondary hypothyroidism: may be associated with impaired synthesis of thyroliberin and/or TSH in pathology of the hypothalamic-pituitary region, including large tumors, traumatic or radiation injury to the pituitary gland, vascular disorders, infectious and infiltrative processes. In certain clinical situations, a TRH stimulation test is used, although it is virtually unused in routine practice.
- Relationship between hypothyroidism and hyperprolactinemia: in $T_3$ and $T_4$ deficiency, the secretion of thyrotropin-releasing hormone increases, which activates pituitary lactotrophs. This leads to hyperprolactinemia.
Class and exam questions
Which group of hormones does thyroliberin belong to?
It belongs to liberins (releasing factors) — hypothalamic peptide neurosecretions. Liberins stimulate the release of tropic hormones in the anterior pituitary gland.
How does thyroliberin travel from the hypothalamus to the pituitary gland?
Neuron axons project to the median eminence, where the neurosecretion is released into the hypophyseal portal system. Through this vascular network, the hormone reaches the cells of the adenohypophysis.
Does thyroliberin affect only the thyroid gland?
No. Thyroliberin acts on the adenohypophysis: it stimulates TSH release, promotes increased prolactin secretion, and stimulates somatotropic cells.
What happens to the level of thyroliberin in hypothyroidism?
In case of a deficiency of thyroid hormones $T_3$ and $T_4$, the secretion of thyroliberin increases.
What else to study on this topic
- Thyroid-stimulating hormone (TSH)
- Iodothyronines (T3 and T4)
- Pituitary portal system
- Negative feedback in endocrinology
- Secondary hypothyroidism
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