Thyroid Gland

• What are the main hormones secreted by the thyroid gland?

  • Thyroxine (T4)
  • Triiodothyronine (T3)
  • Calcitonin (involved in calcium homeostasis)
• What is the primary function of T3 and T4 hormones?

  They regulate basal metabolic rate (BMR) by increasing oxygen consumption and energy expenditure in cells.
• Which hormone is more biologically active: T3 or T4?

  T3 is more biologically active than T4.

  T4 is converted into T3 in peripheral tissues.
• Where are T3 and T4 produced?

  Both are produced in the thyroid follicles from iodinated thyroglobulin.
• What is the role of iodine in thyroid hormone synthesis?

  Iodine is essential for the synthesis of T3 and T4. It is actively transported into the follicular cells and incorporated into tyrosine residues on thyroglobulin.
• Which cells secrete calcitonin, and what is its role?

  Parafollicular cells (C cells) of the thyroid secrete calcitonin, which lowers blood calcium levels by inhibiting osteoclast activity.
• What is the hypothalamic-pituitary-thyroid (HPT) axis?

  A feedback loop:

  • Hypothalamus releases TRH →
  • Pituitary releases TSH →
  • Thyroid produces T3 and T4 →
  • T3/T4 negatively feedback to inhibit TRH and TSH
• What is the role of Thyroid-Stimulating Hormone (TSH)?

  TSH stimulates:

  • Iodine uptake
  • Thyroglobulin synthesis
  • T3/T4 production and secretion
• What is the effect of thyroid hormones on metabolism?

  They increase:

  • Glucose metabolism
  • Lipolysis
  • Protein turnover→ Overall, they increase BMR and heat production.
• List 3 systemic effects of thyroid hormones

  1. Cardiovascular: Increases heart rate and cardiac output
  2. Nervous system: Enhances alertness and reflexes
  3. Growth/development: Essential for brain development and bone growth in children
• What amino acid is essential for thyroid hormone synthesis?

  Tyrosine.
• What element is essential for thyroid hormone synthesis?

  Iodine
• What are the key steps in thyroid hormone synthesis?

  1. Iodide uptake
  2. Oxidation of iodide
  3. Iodination of tyrosine (organification)
  4. Coupling of iodotyrosines
  5. Storage in colloid
  6. Release into bloodstream
• What transporter brings iodide into thyroid follicular cells?

  Sodium-iodide symporter (NIS).
• What enzyme oxidises iodide (I⁻) to iodine (I₂) in the thyroid gland?

  Thyroid peroxidase (TPO).
• What is the role of thyroid peroxidase (TPO) in hormone synthesis?

  It catalyses iodide oxidation, iodination of tyrosine, and coupling of iodotyrosines.
• What protein serves as the scaffold for thyroid hormone synthesis in the colloid?

  Thyroglobulin
• What is formed when one iodine is added to tyrosine on thyroglobulin?

  Monoiodotyrosine (MIT).
• What is formed when two iodines are added to tyrosine on thyroglobulin?

  Diiodotyrosine (DIT).
• What forms T₃ during thyroid hormone synthesis?

  Coupling of one MIT and one DIT.
• What forms T₄ during thyroid hormone synthesis?

  Coupling of two DIT molecules.
• How are thyroid hormones released into circulation?

  Thyroglobulin is endocytosed into follicular cells, proteolysed, and T₃/T₄ are released.
• What plasma proteins bind thyroid hormones for transport?

  Thyroxine-binding globulin (TBG), transthyretin, and albumin.
• Which form of thyroid hormone is biologically active: bound or free?

  Free (unbound) hormone is active.
• What type of receptor do thyroid hormones act on?

  Nuclear receptors (thyroid hormone receptors, TRs).
• What is the mechanism of action of thyroid hormones at the cellular level?

  They bind nuclear receptors → modulate gene expression → alter protein synthesis.
• What are the main physiological actions of thyroid hormones?

  • Increase basal metabolic rate
  • Stimulate protein synthesis
  • Promote growth and development
  • Enhance CNS activity
  • Increase heart rate and cardiac output
• How do thyroid hormones affect metabolism?

  Increase glucose utilisation, lipolysis, and thermogenesis.
• What hormone from the pituitary regulates thyroid hormone secretion?

  Thyroid-stimulating hormone (TSH).
• What hormone from the hypothalamus stimulates TSH release?

  Thyrotropin-releasing hormone (TRH).
• What feedback mechanism controls thyroid hormone levels?

  Negative feedback of T₃/T₄ on the hypothalamus and anterior pituitary.
•  What class of receptors are thyroid hormone receptors (TRs)?

  Thyroid hormone receptors are nuclear receptors that act as ligand-dependent transcription factors.
• Where are thyroid hormone receptors primarily located?

  In the nucleus of target cells, where they regulate gene expression in response to thyroid hormones (mainly T3).
• What are the two main types of thyroid hormone receptors?

  TRα (Thyroid Receptor Alpha) and TRβ (Thyroid Receptor Beta).
• Which genes encode TRα and TRβ?

  • TRα is encoded by the THRA gene.
  • TRβ is encoded by the THRB gene.
• What are the main isoforms of TRα and where are they expressed?

  • TRα1 – widely expressed, especially in heart, skeletal muscle, brain.
  • TRα2 – does not bind T3; may act as a dominant negative inhibitor.
• What are the main isoforms of TRβ and where are they expressed?

  • TRβ1 – expressed in brain, liver, kidney.
  • TRβ2 – mainly in pituitary, hypothalamus, retina, cochlea.
• What hormone primarily binds thyroid hormone receptors?

  T3 (triiodothyronine) is the active hormone that binds TRs with high affinity.T4 (thyroxine) has lower binding affinity.
• What happens to thyroid hormone receptors in the absence of T3?

  They bind DNA at thyroid hormone response elements (TREs) and recruit corepressor proteins, suppressing gene transcription.
• What happens when T3 binds to thyroid hormone receptors?

  Binding of T3 causes a conformational change → corepressors are released, coactivators are recruited, and gene transcription is activated.