The physiology of thyroid hormone production in hypothyroidism is similar to that described in hyperthyroidism. Hypothyroidism may be divided into three types, depending on functional defect. Each of these categories may have various etiologies: (1) primary (primary thyroid T4/T3 secretion defect), (2) secondary (pituitary TSH secretion defect), and (3) tertiary (hypothalamus TRH secretion defect).

From the laboratory standpoint there are three basic laboratory test patterns:

1. Decreased T4 value and markedly elevated TSH value (usually more than 3 times reference range upper limit). This pattern is diagnostic of primary hypothyroidism (with the possible exception of very severe iodine deficiency) and is found in the great majority of primary hypothyroid patients.
2. Thyroxine level in the lower half of the reference range with markedly elevated TSH level. This pattern may be seen in patients with early or mild primary hypothyroidism.
3. Decreased T4 level with decreased TSH values. This pattern suggests secondary or tertiary hypothyroidism.