Thyroid Treatment Prescribed by UK Doctors

The thyroid gland produces thyroxine (T4) and triiodothyronine (T3), hormones that regulate metabolic rate, thermogenesis, cardiac function, bone turnover, and neurological development.

Thyroid dysfunction is among the most common endocrine disorders encountered in UK primary care.

Hypothyroidism (underactive thyroid) affects 2-5% of the general population and up to 10% of women over 60. Primary hypothyroidism accounts for 99% of cases and results from thyroid gland failure.

Hashimoto's thyroiditis (chronic autoimmune thyroiditis) is the predominant cause, responsible for approximately 90% of hypothyroidism in iodine-sufficient countries like the UK.

In Hashimoto's, autoimmune destruction of thyroid follicular cells — mediated by cytotoxic T lymphocytes and anti-thyroid peroxidase (anti-TPO) antibodies — progressively impairs hormone synthesis.

Anti-TPO antibodies are present in 95% of cases and confirm the autoimmune aetiology.

Symptoms of hypothyroidism develop insidiously and are often attributed to ageing, depression, or lifestyle:

• Fatigue and lethargy (most common presenting complaint)
• Weight gain (typically 3-5 kg, due to fluid retention and reduced metabolic rate)
• Cold intolerance
• Constipation
• Dry skin and brittle hair
• Cognitive slowing ("brain fog"), poor concentration, and memory impairment
• Depression and low mood
• Menstrual irregularity (menorrhagia in premenopausal women)
• Bradycardia and elevated diastolic blood pressure
• Myalgia and elevated creatine kinase

Subclinical hypothyroidism — elevated TSH (4.5-10 mU/L) with normal free T4 — affects 5-10% of adults. NICE NG145 recommends treatment if TSH exceeds 10 mU/L, or between 4.

5-10 mU/L if symptoms are present and anti-TPO positive (indicating probable progression to overt hypothyroidism at 5% per year).

Other causes include post-radioiodine or thyroidectomy hypothyroidism, drug-induced (amiodarone, lithium, immune checkpoint inhibitors), and central hypothyroidism (pituitary or hypothalamic disease — rare, requires specialist management).

Levothyroxine sodium is a synthetic form of thyroxine (T4) and the standard replacement therapy for hypothyroidism.

It is one of the most prescribed medications in the UK, with over 30 million prescriptions dispensed annually.

Initiation and dosing follows NICE NG145 and British Thyroid Association (BTA) guidelines:

• Standard starting dose in healthy adults under 65: 50-100 mcg daily (1.6 mcg/kg/day as a rough guide)
• Elderly patients or those with cardiovascular disease: start at 25 mcg daily and increase by 25 mcg every 4-6 weeks
• Recheck TSH 6-8 weeks after initiation or any dose change (steady state requires 5-6 half-lives; T4 half-life is 7 days)
• Target TSH: 0.5-2.5 mU/L for most adults on replacement therapy

Dose optimisation is iterative. Adjustments are made in 25 mcg increments based on TSH response and symptom resolution. Over-replacement (TSH suppressed below 0.

1 mU/L) increases risks of atrial fibrillation (3-fold), osteoporosis (accelerated bone loss), and anxiety. Under-replacement leaves symptoms unresolved and cardiovascular risk elevated.

Absorption factors that affect levothyroxine bioavailability:

• Take on an empty stomach, 30-60 minutes before food or other medications
• Food reduces absorption by 20-40%, and coffee by 30%
• Calcium, iron, and proton pump inhibitors significantly impair absorption if taken concurrently — separate by at least 4 hours
• Consistency of timing matters more than the specific time; some patients prefer bedtime dosing (2-3 hours after last meal), which avoids morning interference

Brand consistency: The BTA advises maintaining the same levothyroxine brand throughout treatment because bioavailability varies by 5-10% between formulations.

Switching brands without TSH rechecking can produce symptomatic fluctuation.

Special populations:

• Pregnancy: levothyroxine requirements increase by 25-50% from the first trimester. TSH should be below 2.5 mU/L in the first trimester. Pre-conception optimisation is critical for foetal neurodevelopment.
• Elderly: lower starting doses and slower titration reduce cardiac risk. TSH target may be relaxed to 1-5 mU/L in very elderly patients.
• Malabsorption (coeliac disease, gastric bypass, IBD): may require higher doses; consider liquid levothyroxine if absorption is inconsistent.

Thyroid replacement therapy requires structured monitoring to ensure dose adequacy, detect overreplacement, and manage the long-term health consequences of thyroid autoimmunity.

Monitoring schedule per NICE NG145:

• TSH every 6-8 weeks during dose titration until stable
• Once stable, annual TSH measurement is sufficient
• Check TSH sooner if symptoms change, new medications are started, pregnancy is confirmed, or significant weight change occurs
• Free T4 is measured alongside TSH to assess adequacy; it should sit in the upper third of the reference range for most patients

Persistent symptoms despite normal TSH: Approximately 5-10% of levothyroxine-treated patients report residual symptoms (fatigue, cognitive difficulty, weight issues) despite TSH in target range.

• Inadequate tissue T3 conversion due to deiodinase polymorphisms (DIO2 gene variants affect T4-to-T3 conversion)
• Coexisting conditions: iron deficiency (ferritin below 30 mcg/L), vitamin D deficiency (below 50 nmol/L), vitamin B12 deficiency, coeliac disease (screen with tTG antibodies), depression
• Autoimmune comorbidity: Hashimoto's patients have higher rates of pernicious anaemia, Addison's disease, type 1 diabetes, and vitiligo

T3 (liothyronine) combination therapy is debated.

The BTA position statement does not recommend routine T3 addition but acknowledges a trial may be considered for patients with persistent symptoms on optimised levothyroxine, under specialist supervision.

Liothyronine is short-acting, expensive (£200-400/month), and difficult to dose consistently.

Thyroid function test interpretation pitfalls:

• Biotin supplements (common in hair/nail products) interfere with immunoassay platforms, causing falsely low TSH and falsely high free T4 — discontinue biotin 3 days before blood testing
• Non-thyroidal illness (sick euthyroid syndrome) suppresses TSH and T3 during acute illness — do not adjust levothyroxine based on tests taken during hospital admission
• Central hypothyroidism produces low free T4 with inappropriately normal or low TSH — TSH alone is insufficient for diagnosis

Annual reviews should also assess cardiovascular risk (hypothyroidism elevates cholesterol by 10-20%, reversible with adequate replacement) and mental health status.

While levothyroxine addresses the hormonal deficiency, lifestyle measures can optimise thyroid function, support autoimmune health, and mitigate symptoms that persist during dose titration.

Nutritional considerations:

• Iodine: the thyroid requires 150 mcg daily for hormone synthesis. UK dietary iodine comes primarily from dairy products, fish, and iodised salt. Deficiency is re-emerging in the UK, particularly in young women and vegans. However, excess iodine supplementation (above 500 mcg/day from kelp tablets) can paradoxically worsen autoimmune thyroiditis (Wolff-Chaikoff effect)
• Selenium 55-200 mcg daily: selenoproteins (glutathione peroxidase, deiodinases) protect thyroid tissue from oxidative damage and facilitate T4-to-T3 conversion. A Cochrane review found selenium supplementation reduces anti-TPO titres by 40% in Hashimoto's patients, though clinical symptom improvement is less consistent
• Iron: ferritin below 30 mcg/L impairs thyroid peroxidase activity and amplifies fatigue. Screen annually and supplement if deficient
• Vitamin D: deficiency (below 50 nmol/L) is more prevalent in autoimmune thyroid disease. Supplementation to 75-100 nmol/L is reasonable, supported by observational data showing inverse correlation with anti-TPO titres

Goitrogens — compounds that interfere with thyroid hormone synthesis — are found in cruciferous vegetables (broccoli, cabbage, kale), soy, and cassava.

In practice, moderate consumption of these foods in a varied diet does not impair thyroid function in individuals on adequate levothyroxine replacement.

Cooking reduces goitrogenic activity by 60-90%.

Exercise: Hypothyroid patients often experience exercise intolerance during undertreated phases.

Once euthyroid on levothyroxine, progressive aerobic and resistance exercise is recommended to address the metabolic deconditioning, weight gain, and muscle weakness that accumulate during hypothyroid periods.

Start with 150 minutes of moderate activity per week and build gradually.

Stress and autoimmunity: Psychological stress activates the hypothalamic-pituitary-adrenal axis and promotes Th1-mediated autoimmune inflammation.

Mind-body practices (meditation, yoga, tai chi) have shown immunomodulatory effects in autoimmune conditions, though thyroid-specific data are limited.

Weight management: The 3-5 kg weight gain typical of hypothyroidism is largely fluid retention that resolves with adequate replacement.

Persistent weight above pre-hypothyroid levels should prompt reassessment of TSH target, exclusion of cortisol excess, and review of dietary and activity patterns.