In a client with primary hypothyroidism, what hormone does the anterior pituitary release?

Thyroid-Stimulating Hormone (TSH) is Released by the Anterior Pituitary in Primary Hypothyroidism

In primary hypothyroidism, the anterior pituitary releases increased amounts of thyroid-stimulating hormone (TSH) in response to low thyroid hormone levels. 1, 2

Pathophysiology of Primary Hypothyroidism

Primary hypothyroidism is characterized by a failure of the thyroid gland to produce adequate thyroid hormones (T3 and T4), which triggers the body's negative feedback system. This condition represents over 99% of all hypothyroidism cases 3.

When thyroid hormone levels decrease:

• The anterior pituitary detects low circulating thyroid hormone levels
• This stimulates the release of increased amounts of TSH
• TSH attempts to stimulate the thyroid gland to produce more thyroid hormones
• In primary hypothyroidism, the thyroid gland cannot respond adequately despite high TSH levels

Diagnostic Findings

The hallmark laboratory findings in primary hypothyroidism include:

• Elevated serum TSH (typically >4.5 mIU/L)
• Low free thyroxine (FT4) levels
• Normal or elevated ACTH levels 1

This pattern distinguishes primary hypothyroidism from secondary (central) hypothyroidism, where TSH would be low or inappropriately normal with low FT4 2.

Clinical Significance

Understanding the elevated TSH response in primary hypothyroidism is crucial because:

• It forms the basis for diagnosis and monitoring of treatment
• TSH levels guide thyroid hormone replacement dosing
• It helps differentiate primary from secondary hypothyroidism 1, 4

Potential Complications

In severe, longstanding primary hypothyroidism with persistently elevated TSH:

• Pituitary enlargement (pituitary pseudotumor) may occur due to thyrotroph hyperplasia
• This represents the pituitary's prolonged attempt to stimulate the thyroid gland
• The enlargement is reversible with appropriate thyroid hormone replacement 5, 6

Common Pitfalls in Diagnosis

When evaluating TSH levels in suspected hypothyroidism:

• Always measure both TSH and FT4 in symptomatic patients to avoid missing central hypothyroidism
• Be aware that low TSH with low FT4 suggests central hypothyroidism, which requires evaluation for hypophysitis or other pituitary disorders
• Remember that primary hypothyroidism (high TSH, low FT4) is far more common than central hypothyroidism 1, 4

In summary, the anterior pituitary releases increased TSH in primary hypothyroidism as part of the body's compensatory mechanism to stimulate thyroid hormone production in a failing thyroid gland.