In primary hypothyroidism, the thyroid gland fails to produce adequate thyroid hormones (T4), leading to low serum T4 levels. The pituitary gland senses this deficiency and responds by increasing secretion of TSH (Thyroid Stimulating Hormone) to stimulate the thyroid. This results in a characteristic pattern of elevated TSH and low T4. This negative feedback mechanism distinguishes primary hypothyroidism from secondary or tertiary causes where TSH may be low or normal.

Reference: Harrison's Principles of Internal Medicine, 20th Edition.