GP · metabolic-and-endocrine-health
Hypothyroidism
Also known as Underactive thyroid · Hashimoto thyroiditis · Myxoedema
GP-fellowship guide to hypothyroidism: TSH-first diagnosis with free T4, subclinical thresholds (treat above 10 mIU/L, consider 4-10 with antibodies or symptoms, lower in pregnancy), levothyroxine dosing at 1.6 µg/kg ideal body weight with elderly/cardiac caution and absorption rules, pregnancy requirements rising ~30% with trimester TSH targets, Cochrane-null T4/T3 combination evidence, over-treatment risks of AF and bone loss, and myxoedema coma emergency overview.
Checked against its sources on 6 Sept 2026
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Red flags
- Hypothermia with altered consciousness in a woman with known or suspected hypothyroidism - myxoedema coma; IV levothyroxine and hydrocortisone now
- Suppressed TSH on two checks in an elderly woman - over-treatment driving atrial fibrillation and bone loss; reduce dose
- Bradycardia with pericardial effusion and falling conscious level - severe hypothyroidism heading to myxoedema coma
- New confusion mimicking dementia in an elderly woman with goitre - check TSH before labelling dementia
Overview
Hypothyroidism is underactive thyroid hormone production. It affects about one in twenty adults, more women than men. Hashimoto thyroiditis is the dominant cause worldwide — autoimmune lymphocytic destruction of the gland. Iatrogenic causes follow: radioactive iodine ablation and thyroidectomy for hyperthyroidism or cancer guarantee lifelong replacement. Drug-induced causes include lithium (inhibits hormone release) and amiodarone (its iodine load blocks hormone synthesis).[4]
Diagnosis starts with TSH. Raised TSH means the pituitary is asking for more hormone. If free T4 is low too, that is overt hypothyroidism and needs treatment. If free T4 sits normal, it is subclinical disease and the decision to treat depends on how high the TSH runs and whether symptoms or antibodies are present. [4]
References7ShowHide
- [1]Jonklaas J, Bianco AC, Bauer AJ, et al. Guidelines for the treatment of hypothyroidism: prepared by the American Thyroid Association task force on thyroid hormone replacement Thyroid, 2014.PMID 25266247
- [2]Alexander EK, Pearce EN, Brent GA, et al. 2017 Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and the Postpartum Thyroid, 2017.PMID 28056690
- [3]Grozinsky-Glasberg S, Fraser A, Nahshoni E, et al. Thyroxine-triiodothyronine combination therapy versus thyroxine monotherapy for clinical hypothyroidism: meta-analysis of randomized controlled trials J Clin Endocrinol Metab, 2006.PMID 16670166
- [4]Surks MI, Ortiz E, Daniels GH, et al. Subclinical thyroid disease: scientific review and guidelines for diagnosis and management JAMA, 2004.PMID 14722150
- [5]Skelin M, Lucijanić T, Amidžić Klarić D, et al. Factors Affecting Gastrointestinal Absorption of Levothyroxine: A Review Clin Ther, 2017.PMID 28153426
- [6]Chen Y, Tai HY Levothyroxine in the treatment of overt or subclinical hypothyroidism: a systematic review and meta-analysis Endocr J, 2020.PMID 32238664
- [7]Sankoda A, Suzuki H, Imaizumi M, et al. Effects of Levothyroxine Treatment on Fertility and Pregnancy Outcomes in Subclinical Hypothyroidism Thyroid, 2024.PMID 38368537