Hypothyroidism etiology

LaFranchi S. Congenital hypothyroidism Etiologies, diagnosis, and management. Thyroid. 199 9 735. 

Assess possible correctable etiologies, including myocardial ischemia, serum potassium concentration (for hyperkalemia), and thyroid function tests (for hypothyroidism). 

Hypothyroidism As replacement or supplemental therapy in hypothyroidism of any etiology, except transient hypothyroidism during the recovery phase of subacute thyroiditis. 

The etiology and pathogenesis of hypothyroidism are outlined in Table 38-5. Hypothyroidism can occur with or without thyroid enlargement (goiter). The laboratory diagnosis of hypothyroidism in the adult is easily made by the combination of a low free thyroxine and elevated serum TSH (Table 38-2). 

Thyroid disorders can be divided into two primary categories conditions that increase thyroid function (hyperthyroidism) and conditions that decrease thyroid function (hypothyroidism).8 There are several different types of hyperthyroidism and hypothyroidism, depending on the apparent etiology, symptoms, and age of onset of each type. The types of hyperthyroidism and hypothyroidism are listed in Table 31-1. Although we cannot review the causes and effects of all the various forms of thyroid dysfunction at this time, this topic is dealt with elsewhere exten-sively.6,8,56 74... 

There are numerous thyroid gland function tests, each designed to determine the etiology of thyroid dysfunction. In general, though, when hypothyroidism is present, circulating T3 and T4 levels are down and TSH is up. The opposite is true of hyperthyroidism. In addition, free (non-protein-bound) T4 and TBG may be determined to clarify inconclusive results. In hyperthyroidism, free T4 is increased but total T4 may be normal. It is the free serum T4 that has been correlated with clinical symptoms rather than total T4. 

PizzuUi A, Ranjbar A. Selenium deficiency and hypothyroidism a new etiology m the differential diagnosis of hypothyroidism in children. Biol Trace Elem Res 2000 77 199-208. 

In hypoosmotic hyponatremia with a normal volume status, the most common etiologies are the syndrome of inappropriate ADH (SIADH), primary polydipsia, hypothyroidism, and adrenal insufficiency (see Figure 46-2). SIADH is usually a result of ectopic or otherwise inappropriate ADH production arising from a variety of conditions (see Chapters 45 and 50) and results in excessive H2O retention. SIADH is often diagnosed when a urine osmolality that is greater than plasma osmolality (usually by more than >i00 mOsmol/kg) is observed in the setting of hyponatremia, but only when renal, adrenal, and thyroid functions are normal. Hypothyroidism impairs free H2O excretion, whereas in adrenal insufficiency, Na" is lost in preference to IC reabsorption. Finally, euvolemic hyponatremia can be... 

Reduced concentrations and availability of T, andTs lead to hypersecretion of pituitary TSH and marked elevations in serum TSH concentrations. The elevated concentration of TSH is an important laboratory finding, particularly in the early detection of thyroid failure. In mild or subclinical hypothyroidism, thyroid hormone concentrations can remain within the normal reference interval, but the TSH concentration is elevated. The etiology of primary hypothyroidism is usually determined on the basis of a good history, a physical examination, and the tests for circulating thyroid autoaiitibodies. 

Kabadi UM. Influence of age on optimal daily levothyroxine dosage in patients with primary hypothyroidism grouped according to etiology. South Med J 1997 90 920-924. 

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... 

Hypothyroidism is quite common in older adults and may present with an indolent course, or it may induce dramatic mental changes such as coma or pericardial effusion with tamponade. The most conunon etiology is primary hypothyroidism, or failure of the thyroid gland to manufacture and release sufficient thyroid hormone. The diagnosis is estabUshed by an elevated thyroid-stimulating hormone (TSH). The treatment is by thyroxine replacement. 

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

There are differences in the etiology of hyper- and hypothyroidism according to the iodine supply to a population. The main causes for hyper- and hypothyroidism in IDA are thyroid autonomy and iodine deficiency, respectively. 

The main etiology of hypothyroidism in middle age and in the elderly in iodine-sufficient areas is autoimmune thyroid... 

Post-operatory hypothyroidism is a consequence of partial or total thyroidectomy in patients with nodular goiters or thyroid tumors. This etiology was found in about one fourth of our patients. Patients with surgical procedures in the neck may also develop thyroid hypofunction, especially in cases in which surgery is combined with radiotherapy (Griffin, 1990). 

Table 106.1 Etiology of hypothyroidism in a group of 655 patients older than 55 years...

Note-. Figures in brackets indicate the percentage of each etiological group in relation to the number of patients with overt or subclinical hypothyroidism or the total number of patients. Source Data from Dfez (2002). Reproduced with permission of the Gerontological Society of America. 

P < 0.001 (x analysis) for the distribution of etiologies in groups of overt and subclinical hypothyroidism. 

Figure 113.1 Etiology of maternal hypothyroidism and its repercussion on the fetus. Iodine defioienoy and autoimmune thyroiditis are the main causes of maternal hypothyroidism. The iodine deficiency can affect both mother and fetus. TSBAb, TSH receptor inhibiting antibodies 131 radioiodine.

In addition, there is now a keen controversy over the possible role of thyroid-growth-blocking antibodies of maternal origin in the etiology of sporadic congenital hypothyroidism (39-41). 

Finally, the role of antithyroglobulin antibodies in the etiology of endemic goiter has been suspected in Argentina (42) but has never been conclusively confirmed, either in endemic goiter and cretinism (43) or in sporadic congenital hypothyroidism (44). 

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