## **Core Concept**
The question tests the understanding of thyroid hormone physiology and the changes that occur in hyperthyroidism. Hyperthyroidism is a condition characterized by excessive production of thyroid hormones, which can lead to various systemic effects. The thyroid hormones, primarily **T3 (Triiodothyronine)** and **T4 (Thyroxine)**, play crucial roles in regulating metabolism.

## **Why the Correct Answer is Right**
In hyperthyroidism, the thyroid gland secretes excess thyroid hormones, **T3** and **T4**, into the bloodstream. These hormones are bound to proteins in the plasma, primarily **Thyroxine-Binding Globulin (TBG)**, but also to albumin and transthyretin. The excess **T4** is the most direct and commonly measured indicator of hyperthyroidism. The body converts **T4** to **T3**, which is the more active form of thyroid hormone. Therefore, the plasma of a hyperthyroid patient contains an excess of **T4**, which is the correct answer.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because while **T3** levels are also elevated in hyperthyroidism, the question seems to point towards the primary hormone in excess, which is **T4**.
- **Option B:** This option might refer to **Thyroid-Stimulating Hormone (TSH)**, which is actually decreased in hyperthyroidism due to negative feedback from high levels of **T3** and **T4** on the pituitary gland.
- **Option C:** This could potentially refer to **Thyroxine Binding Globulin (TBG)**, but TBG levels can be normal or altered due to various factors; it's not directly the excess substance causing hyperthyroidism.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that in hyperthyroidism, **T4** (thyroxine) and **T3** (triiodothyronine) levels are elevated, while **TSH** (Thyroid Stimulating Hormone) levels are suppressed. This suppression of TSH is a critical diagnostic criterion for hyperthyroidism.

## **Correct Answer:** .