MCAT® ExamBiological and Biochemical Foundations of Living SystemsMedium
A patient presents with symptoms including chronic fatigue, cold intolerance, weight gain, and dry skin. Blood tests reveal low levels of thyroid hormones (T3 and T4) and elevated levels of Thyroid-Stimulating Hormone (TSH). Which of the following is the most likely cause of these findings?
- AIodine deficiency leading to goiter
- BSecondary hypothyroidism (pituitary gland dysfunction)
- CPrimary hypothyroidism (thyroid gland dysfunction)
- DHyperthyroidism due to an autoimmune disorder
Show answer & explanationAnswer & explanation
Correct answer: C. Primary hypothyroidism (thyroid gland dysfunction)
Low thyroid hormones (T3/T4) coupled with elevated TSH indicate that the pituitary gland is attempting to stimulate the thyroid, but the thyroid gland itself is unable to produce sufficient hormones. This points to a primary thyroid gland dysfunction, or primary hypothyroidism.
Why the other options are wrong
- A. While iodine deficiency can cause hypothyroidism and goiter, the question specifies 'elevated TSH' and 'low T3/T4', which is consistent with the general mechanism of primary hypothyroidism, regardless of the specific cause like iodine deficiency.
- B. Secondary hypothyroidism (pituitary dysfunction) would result in low TSH, as the pituitary would not be producing enough TSH to stimulate the thyroid.
- D. Hyperthyroidism would involve elevated T3/T4 levels, not low levels.
Thyroid Hormone Regulation
The hypothalamus releases TRH, which stimulates the pituitary to release TSH, which in turn stimulates the thyroid gland to release T3 and T4. T3 and T4 then exert negative feedback on the hypothalamus and pituitary.
- Hypothalamus -> TRH
- Pituitary -> TSH
- Thyroid -> T3 & T4
- Negative feedback by T3/T4 on hypothalamus and pituitary.
Memory trick: Low T3/T4, High TSH = Thyroid's Broken, Pituitary's Pushing!