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 TSH, fT3 and fT4 concentrations is critical in the diagnostics of thyroid gland diseases.