Central hypothyroidism

Exception is central hypothyroidism. THYROID-PITUITARY RELATIONSHIPS... 

Lania A etal Central hypothyroidism. Pituitary 2008 11 181. [PMID 18415684]... 

Adrenal insufficiency can be associated with hypothyroidism (either by autoimmune destruction or due to hypophyseal disease) and carries the risk of acute Addisonian crisis if thyroid substitution precedes glucocorticoid therapy. The diagnostic problem presented by the fact that a few patients with central hypothyroidism have a moderately increased serum TSH should be kept in mind (62). 

Finally, a common, potentially confusing clinical simation should be mentioned. Why are the TSH concentrations suppressed in some patients who are clinically hypothyroid and who have a low free T4 level In patients with long-standing hyperthyroidism, the pituitary thyrotrophs responsible for making TSH become atrophic. The average amount of time required for these cells to resume normal functioning is 6 to 8 weeks. Therefore if a thyrotoxic patient has his or her free T4 concentration lowered rapidly, before the thyrotrophs resume normal function, a period of transient central hypothyroidism will be observed. 

The frequency of secondary or central hypothyroidism in adult patients is clearly lower than that of primary thyroid failure (Table 106.1). This form of hypothyroidism arises from diseases that interfere with the synthesis and release of hypothalamic TRH or pituitary TSH (Rose, 2001 Asteria et ai, 2001). The most common causes are pituitary adenomas and their therapy by surgery or radiotherapy. Less frequent causes of central hypothyroidism are hypothalamic tumors (craniopharyngioma), infiltrative diseases, head trauma, lymphocytic hypophysitis, infections, infarction and metastases (Beck-Peccoz et ai, 1996 Rose, 2001). 

In patients with central hypothyroidism TSH levels may be low because of decreased production, or inappropriately normal or only modestly elevated as a result of synthesis of a TSH molecule with reduced bioactivity (Beck-Peccoz et al, 1985). The development of ultrasensitive TSH assays has made it unnecessary to perform a TRH stimulation test for the diagnosis of most hypothyroid patients. 

LowT4 levels with normal-lowTSH levels are characteristically found in patients with central hypothyroidism. 

In the case of central hypothyroidism, pituitary-adrenal integrity should be evaluated before starting therapy because levothyroxine administration to patients with untreated secondary adrenal insufficiency may precipitate an acute adrenal crisis. Glucocorticoid therapy should be given with T4 if adrenal insufficiency is present. 

The most common causes of central hypothyroidism are pituitary adenomas, craniopharyngiomas and the surgery and/or radiotherapy used to treat them. Central hypothyroidism can also result from other tumors (i.e., glioma, meningioma, chordoma, etc.), from a number of inffltra-tive conditions, such as sarcoidosis and histiocytosis, and from postpartum necrosis (Sheehans syndrome). 

Human exposures with measured concentrations were limited to occupational reports. Symptoms of exposed workers ranged from no adverse health effects to mild discomfort to frank central nervous system effects. Repeated or chronic exposures have resulted in hypothyroidism. Inhalation studies resulting in sublethal effects, such as incapacitation, and changes in respiratory and cardiac parameters were described for the monkey, dog, rat, and mouse lethality studies were available for the rat, mouse, and rabbit. Exposure durations ranged from a few seconds to 24 hours (h). Regression analyses of the exposure duration-concentration relationships for both incapacitation and lethality for the monkey determined that the relationship is C2xt= k and that the relationship for lethality based on rat data is C2 6xt=k. 

Hong, J.-J., Lin, J.-L., Wu, M.-S., Huang, C.-C. Verberckmoes, R. (1996) A chronic glue sniffer with hyperchloraemia metabolic acidosis, rhabdomyolysis, irreversible quadriplegia, central pontine myelinolysis, and hypothyroidism. Nephrol. Dial. Transpl., 11, 1848-1849... 

Secondary hypothyroidism (central thyroid disease) occurs as a result of pituitary or hypothalamic diseases that produce a deficiency in either TSH, TRH, or both. Isolated TSH defi-... 

Less common etiologies include central nervous system lesions that physically compress the pituitary stalk and interrupt tonic hypothalamic dopamine secretion, resulting in hyperprolactinemia. Increased thyroid-releasing hormone (TRH) concentrations in hypothyroidism can stimulate prolactin secretion and cause hyperprolactinemia. During conditions of renal or liver compromise, the... 

Thyroid Hypofunction Hypothyroidism is the most common disorder of thyroid function. It can be divided into patients who have a failure of the thyroid gland to produce sufficient thyroid hormone (primary hypothyroidism) and patients in which pituitary or hypothalamic disease is associated with impaired TSH stimulation (central or secondary hypothyroidism). Worldwide, primary hypothyroidism is caused most often by iodine deficiency. In areas where iodine is sufficient, chronic autoimmune thyroiditis (Hashimoto s thyroiditis) accounts for most cases. Other causes include postpartum thyroiditis, surgical removal, or radioactive iodine ablation of the gland. Hypothyroidism present at birth (cretinism) is the most common preventable cause of mental retardation in the world. 

CHRONIC HEALTH RISKS headaches numbness weakness dizziness tremors loss of appetite effects on central nervous system deafness loss of muscle coordination loss of visual acuity respiratory effects cardiovascular effects cretinism enlarged thyroid gland congenital hypothyroidism in newborns malformation in fetus low fetal body weights in animals. 

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