In primary hypothyroidism, which of the following TSH and free T~4~ patterns is expected?

Explanation

## Thyroid Hormone Regulation and Primary Hypothyroidism ### The Hypothalamic-Pituitary-Thyroid (HPT) Axis ```mermaid flowchart TD A[Hypothalamus]:::outcome -->|TRH| B[Anterior Pituitary]:::outcome B -->|TSH| C[Thyroid Gland]:::outcome C -->|T3, T4| D[Peripheral Tissues]:::outcome D -->|Negative Feedback| A D -->|Negative Feedback| B ``` **Key Point:** In primary hypothyroidism, the thyroid gland itself is dysfunctional. The pituitary and hypothalamus are intact and respond appropriately to low thyroid hormone levels by increasing TSH secretion. ### Pathophysiology of Primary Hypothyroidism 1. **Thyroid dysfunction** (autoimmune, iodine deficiency, post-ablation, etc.) → decreased T~3~ and T~4~ production 2. **Reduced negative feedback** on the pituitary and hypothalamus 3. **Compensatory TSH increase** to stimulate the failing thyroid 4. **Result:** High TSH attempting to normalize low free T~4~ ### Laboratory Patterns in Hypothyroidism | Type | TSH | Free T~4~ | Mechanism | |------|-----|-----------|----------| | **Primary (thyroid failure)** | ↑ Elevated | ↓ Low | Thyroid cannot produce hormone; pituitary responds with ↑ TSH | | **Secondary (pituitary failure)** | ↓ Low | ↓ Low | Pituitary cannot produce TSH; both hormones low | | **Tertiary (hypothalamic failure)** | ↓ Low | ↓ Low | Hypothalamus cannot produce TRH; both hormones low | | **Subclinical primary** | ↑ Elevated | Normal | Early primary hypothyroidism; TSH rises before T~4~ falls | **High-Yield:** The pattern **"High TSH + Low Free T~4~"** is pathognomonic for **primary hypothyroidism**. This is the most commonly tested scenario in NEET PG. **Mnemonic:** **PRIMARY = Pituitary Responds (TSH ↑) when Thyroid fails** — the pituitary is healthy and compensates by raising TSH in an attempt to stimulate the failing thyroid. **Clinical Pearl:** TSH is the most sensitive marker for detecting primary hypothyroidism. Even in subclinical hypothyroidism (normal free T~4~), TSH rises first, making it the earliest sign of thyroid failure.