Anatomy of the HPT axis
What does HPT Axis stand for
Hypothalamic-Pituitary-Thyroid Axis
What are the precursor amino acid and the element needed to synthesise T3/T4?
100points /amino acid
total = 200 points
Tyrosine
Iodine
Which hormone exerts negative feedback on both the hypothalamus and pituitary?
T3
Where do thyroid hormones bind to exert their genomic effects?
Nuclear receptors (thyroid hormone receptors)
What’s a classic symptom of hyperthyroidism related to heat?
Heat intolerance
- sweating, warm etc
Which nucleus in the hypothalamus secretes TRH?
Paraventricular nucleus
Which is more biologically active: T3 or T4?
T3
What happens to TRH and TSH levels in primary hypothyroidism?
Both TRH and TSH increase due to loss of negative feedback.
What is the role of T3 in mitochondrial function?
Increases mitochondrial biogenesis and ATP production
What test is used to confirm Graves' disease?
TSH receptor antibodies (TRAb)
OR
thyroid-stimulating immunoglobulins
What is the name of the portal system that connects the hypothalamus to the anterior pituitary?
Hypophyseal portal system
Which enzyme catalyses the iodination of tyrosyl residues on thyroglobulin?
A) Thyroid peroxidase - TPO
B) Deiodinase 1 - DIO1
C) Deiodinase 2 - DIO2
A) Thyroid peroxidase - TPO
In secondary hyperthyroidism, what is the origin of the dysfunction
Pituitary adenoma secreting excess TSH
Name two tissues where T3 increases Na+/K+ ATPase expression.
Heart and skeletal muscle
Why is TSH low in exogenous thyroxine overdose?
Negative feedback from high T4 suppresses TSH
What cell type in the anterior pituitary secretes TSH?
Thyrotrophs
What is the main storage form of thyroid hormone within the thyroid gland?
- extra 100 if you name where it is found
Thyroglobulin
100 = colloid
In secondary hyperthyroidism, what is the hormone profile?
↑ TSH,
↑ T3/T4
What is the impact of thyroid hormones on cholesterol metabolism?
Increase LDL receptor expression → decrease serum cholesterol
A patient has ↑ T4, ↑ TSH, and no signs of pituitary mass on MRI. What is the most likely diagnosis?
Thyroid hormone resistance syndrome
Describe the anatomical pathway of TSH from secretion to its target.
TSH is secreted from thyrotrophs into the bloodstream, travels to the thyroid gland, binds to TSH receptors on the basolateral membrane of follicular cells, activating cAMP pathways.
Describe the process of peripheral conversion of T4 to T3, including the enzyme and relevant tissues.
T4 is converted to T3 by 5’-deiodinase in peripheral tissues like the liver, kidneys, and skeletal muscle.
A 35-year-old woman presents with fatigue, constipation, and weight gain. Labs show:
TSH: Normal
Free T4: Low
Free T3: Low
What is the likely diagnosis, and why might TSH be normal?
Secondary (central) hypothyroidism — pituitary dysfunction. TSH may be inappropriately normal or low, despite low T4, due to impaired pituitary output.