MCAT® ExamBiological and Biochemical Foundations of Living SystemsEasy
A patient is presenting with symptoms including chronic fatigue, muscle weakness, and a general inability to tolerate cold temperatures. Blood tests reveal low levels of T3 and T4 hormones, and elevated levels of Thyroid-Stimulating Hormone (TSH). Which of the following is the most likely diagnosis?
- ADiabetes mellitus
- BAdrenal insufficiency
- CHypothyroidism
- DHyperthyroidism
Show answer & explanationAnswer & explanation
Correct answer: C. Hypothyroidism
Low T3/T4 and high TSH are classic indicators of hypothyroidism. The symptoms described (fatigue, cold intolerance, muscle weakness) are also consistent with this condition, as thyroid hormones regulate metabolism.
Why the other options are wrong
- A. Diabetes mellitus relates to insulin and glucose regulation, with symptoms like polyuria and polydipsia.
- B. Adrenal insufficiency involves cortisol and aldosterone deficiencies, with different symptoms.
- D. Hyperthyroidism would present with high T3/T4 and low TSH, and symptoms like weight loss, heat intolerance, and anxiety.
Thyroid Hormone Regulation
The regulation of thyroid hormone (T3 and T4) production by the pituitary gland (TSH) and hypothalamus (TRH) via a negative feedback loop.
- Hypothalamus releases TRH.
- Pituitary releases TSH in response to TRH.
- Thyroid gland releases T3 and T4 in response to TSH.
- High T3/T4 inhibit TRH and TSH release.
Memory trick: Low T3/T4, High TSH, Hypo-thyroid's Call