Subclinical Hypothyroidism

Subclinical hypothyroidism is raised TSH with normal free T4.

• Prevalence 4 to 10% of adults, more in older women.
• Causes: early Hashimoto thyroiditis (TPO antibodies positive), post radioiodine or thyroidectomy, drugs (amiodarone, lithium), iodine excess.
• Progression to overt hypothyroidism about 2 to 4% per year, higher with TSH over 10 or positive TPO.
• Associated with dyslipidaemia, increased cardiovascular risk when TSH above 10.

• Mostly asymptomatic; mild fatigue, cold intolerance, constipation.
• Goitre.
• Pregnancy: associated with adverse outcomes.

• Repeat TSH and free T4 after 2 to 3 months to confirm (transient elevations are common, e.g. recovery from illness).
• TPO antibodies, lipid profile.
• TSH above 4.5 to 10 mIU/L (mild) versus above 10 mIU/L.
• Use age specific TSH reference ranges (upper limit up to 6 to 7 in over 70).

• Levothyroxine 25 to 75 micrograms daily (1.0 to 1.6 micrograms/kg) for TSH persistently above 10, pregnancy or planning pregnancy (TSH above 2.5 with positive TPO, above 4 otherwise), symptoms, positive TPO with TSH 7 to 10 in younger patients, goitre, infertility.
• Observation with TSH every 6 to 12 months otherwise, especially over 70 years with TSH under 10.
• Target TSH 0.5 to 2.5 (higher in elderly).
• Avoid overtreatment (atrial fibrillation, bone loss).