Hypothalamic-Pituitary-Thyroid Axis

Rodrigo Arrangoiz MS, MD, FACS, FSSO
  • A full appreciation of thyroid function and its testing:
    • Is dependent on a thorough understanding of its axis of regulation
  • Thyroid stimulating hormone (TSH):
    • The major regulator of thyroid hormone production and secretion:
      • Is synthesized and secreted by the thyrotroph cells of the anterior pituitary
    • The main stimulator of TSH production by the anterior pituitary is:
      • Thyrotropin-releasing hormone (TRH):
        • Via the hypothalamic-pituitary portal system
    • Conversely, exposure to the thyrotrophs by circulating T4 and T3:
      • Inhibits the secretion of TSH and TRH via a negative feedback loop:
        • Decreasing gene expression of these hormones:
          • Therefore decreasing its activity
    • As with other pituitary hormones:
      • TSH secretion is pulsatile in nature:
        • With higher levels seen at night than during the day
      • However, despite this diurnal variation:
        • Serum TSH concentrations generally remain in the reference range when drawn during the day:
          • But may be elevated if  drawn at night
    • Among TRH and thyroid hormone itself:
      • TSH secretion is also affected by:
        • Glucocorticoids
        • Retinoids
        • Somatostatin
        • Dopamine
      • Consequently, several disease states and medications can affect TSH levels such as:
        • Pituitary or hypothalamic dysfunction
        • Recent hyperthyroidism
        • Critical illness
        • Starvation
        • Use of certain medications:
          • Beta Blockers
          • Ipodate
          • Amiodarone
          • Dexamethasone
          • Prophylthiouracil
        • Interference with serum thyroid autoantibodies
        • Thyroid hormone resistance syndromes
      • During starvation and acute illness:
        • Expression of the 5 deiodinase type 3 (Dio3) is increased:
          • Converting the bioactive T4 and T3 to two biologically inactive molecules:
            • Reverse T3 (rT3) and 3,3′-diiodothyronine (T2)
  • Given this, thyroid physiology can be affected in nonthyroidal illness (euthyroid sick syndrome) and it is important that the diagnosis of primary thyroid dysfunction not be established during severe illness based solely on an abnormal serum TSH:
    • In these conditions, serum TSH concentrations may be low, normal, or high, due to the TSH-lowering effects of medications or from an acquired central hypothyroidism and therefore, when possible, assessment of thyroid function should be done after recovery from an acute illness
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Published by Rodrigo Arrangoiz MS, MD, FACS, FSSO

My name is Rodrigo Arrangoiz I am a breast surgeon/ thyroid surgeon / parathyroid surgeon / head and neck surgeon / surgical oncologist that works at Center for Advanced Surgical Oncology in Miami, Florida. I was trained as a surgeon at Michigan State University from (2005 to 2010) where I was a chief resident in 2010. My surgical oncology and head and neck training was performed at the Fox Chase Cancer Center in Philadelphia from 2010 to 2012. At the same time I underwent a masters in science (Clinical research for health professionals) at the University of Drexel. Through the International Federation of Head and Neck Societies / Memorial Sloan Kettering Cancer Center I performed a two year head and neck surgery and oncology / endocrine fellowship that ended in 2016. Mi nombre es Rodrigo Arrangoiz, soy cirujano oncólogo / cirujano de tumores de cabeza y cuello / cirujano endocrino que trabaja Center for Advanced Surgical Oncology en Miami, Florida. Fui entrenado como cirujano en Michigan State University (2005 a 2010 ) donde fui jefe de residentes en 2010. Mi formación en oncología quirúrgica y e n tumores de cabeza y cuello se realizó en el Fox Chase Cancer Center en Filadelfia de 2010 a 2012. Al mismo tiempo, me sometí a una maestría en ciencias (investigación clínica para profesionales de la salud) en la Universidad de Drexel. A través de la Federación Internacional de Sociedades de Cabeza y Cuello / Memorial Sloan Kettering Cancer Center realicé una sub especialidad en cirugía de cabeza y cuello / cirugia endocrina de dos años que terminó en 2016.

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