Approach to abnormal TFTs
- Start with the pattern: low FT4 + high TSH = primary hypothyroidism; normal FT4 + high TSH = subclinical hypothyroidism; suppressed TSH + high FT4/FT3 = hyperthyroidism.
- Confirm and contextualise: repeat if mild/discordant, check meds (amiodarone, lithium, biotin supplements), intercurrent illness, and pregnancy status.
- Autoimmunity: in adults with subclinical hypothyroidism, consider measuring thyroid peroxidase antibodies (TPOAbs) once (do not repeat routinely).
Primary hypothyroidism: treating and dosing (NICE)
First line: levothyroxine monotherapy. Do not routinely use liothyronine (alone or combined) for primary hypothyroidism.
- Adults <65 with no cardiovascular disease: consider starting ~1.6 micrograms/kg/day, rounded to the nearest 25 micrograms.
- Adults ≥65 or with cardiovascular disease: start 25–50 micrograms/day and titrate.
- Targets and safety: aim for TSH within reference range; avoid TSH suppression/thyrotoxicosis.
Monitoring (practical schedule)
- TSH kinetics: in very high baseline TSH or long untreated disease, TSH may take up to 6 months to normalise — account for this when adjusting.
- Adults on levothyroxine: consider TSH every 3 months until stable (2 similar in-range results 3 months apart), then annually.
- Persistent symptoms: consider FT4 alongside TSH if symptoms persist after starting treatment.
Subclinical hypothyroidism (key thresholds)
- TSH ≥10 mIU/L: consider levothyroxine if confirmed on 2 occasions 3 months apart.
- TSH above reference but <10 mIU/L: consider a 6-month trial of levothyroxine in adults <65 with symptoms, confirmed on 2 occasions 3 months apart.
- If no benefit: re-check TSH, adjust if still raised; if symptoms persist with in-range TSH, consider stopping and follow NICE monitoring advice for untreated subclinical disease.
Frequently asked questions
Do I need to “treat the number” if the patient feels well?
Not always. NICE is explicit that routine treatment of subclinical hypothyroidism depends on repeated thresholds and clinical context. Use TSH ≥10 (confirmed) as the key decision point, and consider symptomatic trials in under-65s with persistent symptoms and TSH <10.
How fast can I titrate levothyroxine?
Adjust based on TSH response and clinical risk. In older patients or those with cardiovascular disease, use slower titration and lower increments. Repeat TSH at appropriate intervals (often 6–8 weeks after dose changes, then per NICE stability schedule).
When should I seek specialist advice?
Use specialist input for complex patterns (discordant TFTs), suspected central hypothyroidism, pregnancy with difficult control, thyroid enlargement/nodules with concerning features, or subclinical hyperthyroidism with persistently suppressed TSH.
Transparency
This page is an educational, clinician-written summary of publicly available NICE guidance intended for trained healthcare professionals. It uses original wording (not copied text) and should be used alongside the full NICE source, local pathways, and clinical judgement. Doses provided are for general reference; always check the BNF/SPC.