**Core Concept**
Electroconvulsive therapy (ECT) is a psychiatric treatment that involves the application of electrical impulses to induce a controlled seizure in patients with severe mental illnesses, such as depression, mania, or catatonia. The underlying mechanism of ECT is thought to involve changes in neurotransmitter levels and synaptic plasticity, although the exact mechanisms are not fully understood.

**Why the Correct Answer is Right**
The correct answer is related to the effects of ECT on the brain. During ECT, the electrical impulses induce a seizure that can lead to changes in the brain's neurotransmitter levels, particularly an increase in the release of norepinephrine, serotonin, and acetylcholine. This increase in neurotransmitter release can lead to an improvement in mood and a reduction in symptoms of depression. Additionally, ECT has been shown to increase the expression of brain-derived neurotrophic factor (BDNF), which is involved in neuronal growth and survival.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because ECT does not primarily cause changes in blood pressure or heart rate, although it may lead to transient increases in these parameters during the procedure.
**Option B:** This option is incorrect because ECT does not primarily cause changes in blood glucose levels, although it may lead to transient changes in glucose metabolism during the procedure.
**Option C:** This option is incorrect because ECT does not primarily cause changes in thyroid function, although it may lead to transient changes in thyroid hormone levels during the procedure.

**Clinical Pearl / High-Yield Fact**
ECT is a highly effective treatment for severe depression, with response rates ranging from 50-90% in some studies. However, the exact mechanisms of ECT are not fully understood, and further research is needed to elucidate its effects on the brain and nervous system.

**Correct Answer: C. Increased expression of brain-derived neurotrophic factor (BDNF)**