Hypothalamicpituitarythyroid axis

Thyroid Factor

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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:

y i Basal serum TSH: a normal level virtually rules out thyrotoxicosis or hypothyroidism;

° 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

secretion

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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