The hypothalamic-pituitary-thyroid axis is an integrated organ complex providing for the regulated production of thyroid hormones. It is affected by the following factors: Hypothalamic TRH production: Increased: dopamine, noradrenaline Decreased: T3, T4, glucocorticoids Pituitary TSH production: Increased: TRH
Decreased: dopamine, oestrogens, T3, T4, glucocorticoids Thyroid gland hormone production: Increased: TSH, iodine, human chorionic gonadotrophin (HCG), catechol-amines
Thyroid function tests e Serum thyroid hormones: T4, free T4, T3, free T3, thyroglobulin.
h Tests of the thyroid-pituitary axis:
° Serum TSH response to exogenous TRH.
Dynamic tests of thyroid activity:
In vivo radionuclide uptake studies: 99Tcm, 123I, 131I; T3 suppression tests; TSH stimulation test.
Table 10.1 Hypothalamic-pituitary-thyroid axis dysfunction
TSH response to TRH
Primary hypothyroidism Secondary hypothyroidism (pituitary failure) Tertiary hypothyroidism (hypothalamic failure) Primary hyperthyroidism Secondary hyperthyroidism
Low High Exaggerated response
Low Low No increase in TSH
Low Low Normal/exaggerated TSH
High Low Blunted or no response
High/normal High Normal
Decreased: iodine, medications (lithium; anti-thyroid medication); dietary goitrogens
Thyroid stimulating hormone is a glycoprotein, with an alpha chain of 89 amino acids and a beta chain of 112 amino acids.
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