Hypothalamus hypothyroidism

Prolactin is an essential hormone for normal production of breast milk following childbirth. It also plays a pivotal role in a variety of reproductive functions. Prolactin is regulated primarily by the hypothalamus-pituitary axis and secreted solely by the lactotroph cells of the anterior pituitary gland. Under normal conditions, secretion of prolactin is predominantly under inhibitory control by dopamine and acts on the D2 receptors located on the lactotroph cells. Increase of hypothalamic thyrotropin-releasing hormone (TRH) in primary hypothyroidism can stimulate the release of prolactin. 

Tertiary hypothyroidism, or hypothalamic hypothyroidism, results from impaired TRH stimulation of pituitary TSH. This may be due to a disorder that damages the hypothalamus or interferes with hypothalamic-pituitary portal blood flow, thereby preventing delivery of TRH to the pituitary. Tumors, trauma, radiation therapy, or infiltrative disease of the hypothalamus can cause such damage. This relatively rare form of hypothyroidism is also characterized by inappropriately low levels of serum TSH. 

Thyroid disorders may be divided into over- and underproduction of the thyroid hormones. These may be caused by thyroid gland disorders or disorders of the pituitary gland (TSH production) or hypothalamus (thyrotropin-releasing hormone release). Thyroid hormone deficiency in infancy may cause mental retardation if it is not corrected immediately after birth. For this reason, many states require thyroid function tests in all newborns. In adults, thyroid deficiency may be caused by Hashimoto s thyroiditis, an immune disorder, or dietary iodine deficiency, in which case it is called simple goiter. The term "myxedema" has been used to refer to hypothyroidism of whatever cause. Myxedemas may... 

Secondary hypothyroidism Either the hypothalamus is unable to release sufficient quantities of thyrotropin-releasing hormone or the anterior pituitary gland is unable to release sufficient thyroid-stimulating hormone. 

Three possible causes of hypothyroidism are the decreased secretion of thyrotropin-releasing hormone (TRH) by the hypothalamus gland, decreased secretion of thyroid-stimulating hormone (TSH) by the pituitary gland, and decreased secretion of thyroid hormones by the thyroid gland. 

Etiology 1) Primary hypothyroidism - surgery, radioiodine ablation, thyroiditis. 2) Secondary hypothyroidism - hypofunction of pituitary or hypothalamus. 

Interestingly, TH also upregulates NOS activity at the hypothalamus and kidney levels, where the system is involved in sodium balance and arterial pressure regulation by limiting salt appetite and favoring sodium excretion, respectively. Therefore, besides the above-mentioned renal hemodynamic and endocrine factors, this mechanism may also contribute to the salt sensitivity of arterial BP in hypothyroid individuals (Quesada et ai, 2002). 

A patient with clinical hypothyroidism who does not have an elevated serum TSH concentration must have hypothyroidism secondary to either hypothalamic or pituitary disease. A finding of an undetectable TSH level per se does not distinguish these patients from normal because of the insensitivity of the assay. A TRH stimulation test may identify the site of the primary dysfunction, whether pituitary or hypothalamus (see Section 5.3). 

Strbak, V. and Greer, M.A. (1981) Thyrotropin secretory response to thyrotropin releasing hormone in the hypothyroid perinatal rat Further evidence of thyrotroph independence of the hypothalamus during early ontogenesis. Endocrinology 108 1403. 

In determining the locus of the biochemical lesion leading to the hypothyroid state in vitamin deficiency, various possibilities such as primary with a defective thyroid gland, secondary with a defective pituitary thyrotroph, or tertiary with a defective hypothalamus were considered. The chronic deficiency of TRH in the deficient rat is indicated by an increase in the number of TRH receptors with no change in receptor affinity. Results reported (12) are consistent with a hypothalamic type of hypothyroidism in the vitamin Bg-deficient rat caused by the specific decrease in hypothalamic serotonin level. 

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