- Hypothyroidism can significantly impact glycemic control in patients with diabetes primarily because it decreases the overall metabolic rate.
- A lower metabolic rate results in reduced energy expenditure, which in turn can affect how the body uses glucose.

- Additionally, hypothyroidism is known to reduce insulin sensitivity. This means that the body's cells become less responsive to insulin, making it harder for glucose to enter the cells and be used for energy. Consequently, blood glucose levels may rise, contributing to deterioration in glycemic control.

- In contrast, options such as increased hepatic gluconeogenesis or enhanced insulin secretion are more commonly associated with hyperthyroidism or other conditions rather than hypothyroidism.

- Therefore, the key mechanisms by which hypothyroidism worsens glycemic control are through decreased metabolic rate and reduced insulin sensitivity, which explain the clinical observation of harder-to-control blood sugar levels in diabetic patients with hypothyroidism.

Reference: Harrison's Principles of Internal Medicine, 20th Edition, Endocrinology Chapter, Page 2412