The hormones free triiodothyronine (fT3), free thyroxine (fT4), calcitonin and TSH (thyroid stimulating hormone,) are closely involved in the regulation of the human organism via the hypothalamus, pituitary gland and thyroid gland. The binding of TSH to the TSH receptors of the thyroid gland causes the synthesis and secretion of the T3 and T4 hormones. The largest proportion of circulating T3 and T4 is bound. Only free T3 and free T4 are biologically available.
In primary thyroid overfunction the thyroid gland produces too much T3 and T4, which inhibits the pituitary gland from releasing TSH. The TSH concentration in blood decreases; FT4 and FT3 are borderline high or increased. In secondary thyroid overfunction FT3 and FT4 are increased. The defect lies in the pituitary gland. It may release too much TSH– because of a TSH-producing tumour, which stimulates the thyroid gland to produce more T3 and T4. The cause may also be a thyroid hormone resistance. As well as being caused by disturbances in thyroid hormone regulation, the symptoms of a hyper- or hypothyroidism can also be the result of thyroiditis (inflammation of the thyroid gland).
Alongside the observation of the clinical disease signs, the determination of fT3 and fT4 concentrations, among others, is critical in the diagnostics of thyroid gland diseases.
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