What is the relationship between plasma Thyroid Stimulating Hormone (TSH) concentration and basal metabolic rate during blockade of the hypothalamohypophysial portal vessels?

Relationship Between Plasma TSH and Basal Metabolic Rate During Hypothalamohypophysial Portal Vessel Blockade

The correct answer is C: Decreased plasma TSH and decreased basal metabolic rate.

Physiological Basis

When the hypothalamohypophysial portal vessels are blocked, the communication pathway between the hypothalamus and anterior pituitary is disrupted. This has significant consequences for the hypothalamic-pituitary-thyroid (HPT) axis:

1. Disruption of TRH Delivery:

   • Thyrotropin-releasing hormone (TRH) from the paraventricular nuclei of the hypothalamus cannot reach the anterior pituitary 1
   • Without TRH stimulation, TSH secretion from the pituitary is significantly reduced 2

2. Decreased TSH Production:

   • TRH is the primary positive regulator of TSH synthesis and release 1
   • Studies show that lesions in the paraventricular nuclei cause approximately 60% drop in plasma TSH concentration 2
   • Without portal vessel communication, this stimulatory signal is lost

3. Reduced Thyroid Hormone Production:

   • Lower TSH levels lead to decreased stimulation of the thyroid gland
   • This results in reduced production and secretion of thyroid hormones (T3 and T4)

4. Impact on Basal Metabolic Rate:

   • Thyroid hormones are primary regulators of basal metabolic rate 3
   • Decreased thyroid hormone levels lead directly to decreased basal metabolic rate
   • This relationship is well-established in clinical settings where BMR was historically used to determine thyroid status 3

Clinical Correlation

The relationship between thyroid function and metabolic rate is fundamental to understanding this physiological cascade:

• Thyroid hormones increase basal metabolic rate through multiple mechanisms:

  • Increased Na+/K+ ATPase activity
  • Enhanced mitochondrial function and oxygen consumption
  • Increased heat production
  • Accelerated protein turnover

• When thyroid hormone levels fall due to decreased TSH stimulation, these metabolic processes slow down, resulting in:

  • Decreased oxygen consumption
  • Reduced heat production
  • Lower overall energy expenditure 4

Differential Analysis of Answer Options

Let's analyze why the other options are incorrect:

• Option A (Decreased TSH and increased BMR): Contradicts the physiological cascade, as decreased TSH leads to decreased thyroid hormone production, which cannot increase BMR.

• Option B (Increased TSH and decreased BMR): Incorrect because portal vessel blockade prevents TRH from reaching the pituitary, which would decrease, not increase, TSH production.

• Option D (Increased TSH and increased BMR): Doubly incorrect, as portal vessel blockade decreases TSH, and even if TSH were increased, it would take time for this to translate into increased thyroid hormone production and subsequent BMR elevation.

Common Pitfalls to Avoid

1. Confusing direct vs. indirect effects: Remember that TSH itself doesn't directly affect BMR; it works through stimulating thyroid hormone production.

2. Timing considerations: While the question asks about the relationship during blockade, it's important to understand that metabolic effects of thyroid hormone changes may take hours to days to fully manifest.

3. Forgetting negative feedback: Under normal conditions, thyroid hormones exert negative feedback on both the hypothalamus and pituitary. When portal vessels are blocked, this feedback loop is disrupted at the hypothalamic level, but direct feedback to the pituitary remains intact, further suppressing any residual TSH production.

Understanding this physiological cascade is crucial for interpreting thyroid function tests and managing disorders of the hypothalamic-pituitary-thyroid axis in clinical practice.