The correlation between levels of the pituitary hormone and the target organ hormone will help to establish the cause of underactivity in a target gland (Fig. 2.21); for example, in a patient with hypothyroidism, low or normal TSH levels combined with low T4 levels imply pituitary gland malfunction or pituitary hypothyroidism. High TSH concentrations combined with a low T4 level indicate disease in the thyroid gland itself (that is, primary hypothyroidism).
