What gland secretes Thyroid Stimulating Hormone (TSH), the thyroid leading hormone?

Thyroid Stimulating Hormone (TSH) Secretion

Thyroid Stimulating Hormone (TSH) is secreted by the anterior pituitary gland, not the thyroid gland. 1, 2

Anatomical and Physiological Basis

The hypothalamic-pituitary-thyroid axis consists of two separate neuroendocrine organs:

• Anterior pituitary system: Secretes thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), prolactin, growth hormone (GH), and gonadotropins 1
• Posterior pituitary system: Consists of axonal terminations of neurons from the hypothalamus, secreting oxytocin and vasopressin/antidiuretic hormone 1

TSH specifically:

• Is a glycoprotein hormone produced and released by the anterior pituitary gland 2
• Has the highest density of T3 receptors in the pituitary gland 3
• Is released in a pulsatile pattern regulated by thyrotropin-releasing hormone (TRH) from the hypothalamus 4

Regulation of TSH Secretion

TSH secretion is tightly regulated through multiple mechanisms:

1. Hypothalamic control: TRH from the hypothalamus stimulates TSH release from the anterior pituitary 4
2. Negative feedback: Thyroid hormones (T3 and T4) inhibit TSH secretion through negative feedback on the pituitary 3, 5
3. Ultra-short loop regulation: Evidence suggests that the pituitary may contain TSH receptors on folliculo-stellate cells, allowing it to sense its own TSH release 6

Clinical Significance

Understanding the source of TSH secretion is critical for:

• Diagnosing thyroid disorders:

  • Primary hypothyroidism: High TSH, low T4
  • Central (secondary) hypothyroidism: Low/normal TSH, low T4 7

• Monitoring thyroid function:

  • In primary thyroid disorders, TSH is the most sensitive marker
  • In central hypothyroidism, free T4 levels in the upper half of the reference range should be targeted 7

• Preventing diagnostic errors:

  • TSH secretion can be affected by factors other than thyroid disorders, including acute illness, medications, pregnancy, and pituitary disorders 1
  • Serial TSH measurements are essential to establish persistent thyroid dysfunction 1

Common Pitfalls in TSH Assessment

• Relying on a single TSH value for diagnosis (values can vary by up to 50% day-to-day) 1
• Not considering factors that affect TSH secretion (medications, acute illness, etc.) 1
• Using inappropriate reference ranges for different populations (e.g., elderly patients may have naturally higher TSH levels) 1
• Failure to recognize central hypothyroidism (low/normal TSH with low free T4) 7

In conclusion, TSH is produced by the anterior pituitary gland as part of the hypothalamic-pituitary-thyroid axis, and its secretion is regulated by multiple complex mechanisms including hypothalamic TRH stimulation and negative feedback from thyroid hormones.