Which of the following laboratory findings is characteristic of primary hypothyroidism?

Explanation

## Laboratory Diagnosis of Primary Hypothyroidism **Key Point:** Primary hypothyroidism is characterized by **elevated TSH and low free T~4~** — the hallmark pattern that distinguishes it from secondary (central) hypothyroidism. ### Pathophysiology of TSH Elevation 1. Thyroid gland fails to produce adequate T~3~ and T~4~ 2. Circulating free T~4~ falls below normal 3. Loss of negative feedback on the anterior pituitary 4. Pituitary responds by increasing TSH secretion in an attempt to stimulate the failing thyroid 5. Result: **High TSH, Low free T~4~** **Mnemonic:** **PRIMARY = Pituitary Responds In Reverse to Minimal thyroid output** — when the thyroid fails, TSH goes up (not down). ### Diagnostic Algorithm for Hypothyroidism ```mermaid flowchart TD A[Suspect hypothyroidism<br/>Fatigue, weight gain, cold intolerance]:::outcome --> B[Check TSH + free T4]:::action B --> C{TSH elevated?}:::decision C -->|Yes| D{Free T4 low?}:::decision D -->|Yes| E[Primary hypothyroidism]:::outcome D -->|No| F[Subclinical hypothyroidism]:::outcome C -->|No| G{Free T4 low?}:::decision G -->|Yes| H[Central hypothyroidism]:::outcome G -->|No| I[Euthyroid]:::outcome ``` ### Laboratory Patterns in Hypothyroid States | Condition | TSH | Free T~4~ | Clinical Significance | |-----------|-----|-----------|----------------------| | **Primary hypothyroidism** | ↑↑ | ↓ | Thyroid failure; requires treatment | | Subclinical hypothyroidism | ↑ | Normal | Early stage; may progress | | Central hypothyroidism | ↓ or normal | ↓ | Pituitary/hypothalamic disease | | Secondary hypothyroidism | ↓ | ↓ | Pituitary insufficiency | | Tertiary hypothyroidism | ↓ | ↓ | Hypothalamic insufficiency | **High-Yield:** TSH is the **most sensitive screening test** for primary hypothyroidism. A single elevated TSH with low free T~4~ is diagnostic; you do NOT need additional tests (anti-TPO, anti-thyroglobulin) to confirm primary hypothyroidism — those are useful for etiology (Hashimoto's) but not diagnosis. **Clinical Pearl:** In subclinical hypothyroidism (elevated TSH, normal free T~4~), treatment decisions depend on TSH level, symptoms, and anti-TPO antibody status. Do not over-treat mild elevation without clinical indication.