## Investigation Strategy in Suspected Hypothyroidism ### Clinical Context The patient presents with classic hypothyroid symptoms (fatigue, weight gain, cold intolerance, bradycardia, dry skin) and has an elevated TSH. The next step is to confirm primary hypothyroidism by measuring free T4. ### Why Free T4 is the Investigation of Choice **Key Point:** In primary hypothyroidism, TSH rises as the pituitary attempts to stimulate a failing thyroid gland. Free T4 will be low or low-normal, confirming the diagnosis. **High-Yield:** The diagnostic algorithm is: 1. Elevated TSH → measure free T4 2. If free T4 is low → primary hypothyroidism confirmed 3. If free T4 is normal → subclinical hypothyroidism ### Diagnostic Confirmation | Investigation | Interpretation in Primary Hypothyroidism | | --- | --- | | TSH | Elevated (>4.0 mIU/L) | | Free T4 | Low or low-normal (<12 pmol/L or <0.9 ng/dL) | | TPO antibodies | Positive in autoimmune thyroiditis (Hashimoto's) | | Thyroid ultrasound | Shows hypoechoic gland in autoimmune disease; not diagnostic | | Thyroid uptake scan | Rarely used; shows low uptake in primary hypothyroidism | **Clinical Pearl:** Free T4 (not total T4) is preferred because it is unaffected by thyroid-binding globulin levels and reflects the biologically active hormone. ### Role of Other Investigations - **TPO antibodies:** Useful to determine *etiology* (autoimmune vs. iodine deficiency vs. drugs) *after* diagnosis is confirmed, but not for initial confirmation. - **Thyroid ultrasound:** Helpful in assessing nodules or structural disease, but not required for diagnosis of simple hypothyroidism. - **Thyroid uptake scan:** Rarely used in modern practice; reserved for specific scenarios (thyroiditis, iodine deficiency assessment).
Sign up free to access AI-powered MCQ practice with detailed explanations and adaptive learning.