GP · metabolic-and-endocrine-health
Thyroid function interpretation
Also known as TSH interpretation · Thyroid function tests · Subclinical hypothyroidism · Levothyroxine management
GP-fellowship guide to thyroid function interpretation: TSH-first testing strategy, the interpretation patterns from raised TSH to central hypothyroidism, subclinical hypothyroidism management (when to treat and when to watch), levothyroxine titration with 6-8-week monitoring, pregnancy-specific TSH ranges and dose increases, and the lower starting dose for elderly patients and those with ischaemic heart disease.
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Red flags
- A low TSH with a LOW free T4 suggests central hypothyroidism (pituitary or hypothalamic) — do not start levothyroxine without excluding pituitary pathology
- Start levothyroxine at a LOWER dose in elderly patients and those with ischaemic heart disease — full replacement doses can precipitate angina or arrhythmia
- Pregnancy changes everything: TSH reference range drops, levothyroxine requirements increase by 30-50%, and untreated hypothyroidism harms fetal neurodevelopment
- Do not screen asymptomatic patients for hypothyroidism — targeted testing only for high-risk groups
Overview
Thyroid function testing is one of the most common investigations in general practice. TSH is the first-line test; free T4 is added when TSH is abnormal. The interpretation patterns follow a logical framework once you understand the hypothalamic-pituitary-thyroid axis. Hashimoto thyroiditis is the cause of primary hypothyroidism in up to 85% of patients in iodine-replete areas.[1]
References3ShowHide
- [1]Chaker L, Papaleontiou M Hypothyroidism: A Review JAMA, 2025.PMID 40900603
- [2]McCahon D, Haque MS, Parle J, et al. Subclinical thyroid dysfunction symptoms in older adults: cross-sectional study in UK primary care Br J Gen Pract, 2020.PMID 31932293
- [3]Holley M, Razvi S, Farooq MS, et al. Cardiovascular and bone health outcomes in older people with subclinical hypothyroidism treated with levothyroxine: a systematic review and meta-analysis Syst Rev, 2024.PMID 38720372