Thyroid function test

Thyroid Results

TSH (Reference range: 0.35-4.94 mU/L)

Free T4 (Reference range: 9.0-19.1 pmol)

 

What is Hypothyroidism?

Hypothyroidism is a condition where the thyroid gland fails to produce enough thyroid hormone, resulting in a raised TSH and a low or low-normal free T4. Subclinical hypothyroidism refers to elevated TSH with normal T3/T4.

Common Causes

  • Autoimmune thyroiditis (Hashimoto’s)
  • Post-radioiodine therapy or thyroidectomy
  • Congenital hypothyroidism
  • Medications (e.g. amiodarone, lithium)
  • Destructive thyroiditis (e.g. postpartum, subacute)
  • Iodine deficiency (rare in UK)

Stepwise Clinical Approach

Step 1: Interpret TSH

  • TSH 5 - 10 mU/L with normal FT4: subclinical hypothyroidism
  • TSH > 10 mU/L or low FT4: overt hypothyroidism - treatment indicated

Step 2: Manage Subclinical Hypothyroidism

  • Repeat TSH and FT4 in 3 - 6 months to confirm persistence
  • Consider treatment if:
    • TSH > 10
    • Patient is symptomatic
    • Has cardiovascular disease, osteoporosis or is age > 60
    • Pregnant or trying to conceive
  • Check TPO antibodies - raised titres support autoimmune cause
  • If TPO negative and asymptomatic, monitor annually

Step 3: Treat Overt Hypothyroidism

  • Start levothyroxine 50 - 100 mcg once daily
  • Increase by 25 - 50 mcg every 3 - 4 weeks
  • If age > 50 or cardiac disease: start with 25 mcg once daily
  • Maintenance dose typically 100 - 200 mcg once daily

Step 4: Monitor and Adjust

  • Check TSH and FT4 every 6 - 8 weeks until within normal range
  • Once stable, monitor annually
  • Adjust dose according to TSH and symptoms (TSH is the main guide)

Step 5: When to Refer

  • Age under 16
  • Pregnant or trying to conceive
  • Goitre or nodules present
  • Unresponsive to adequate dosing (TSH not normal & patient symptomatic)
  • Symptoms of pituitary disease - consider secondary hypothyroidism
  • TSH remains high despite levothyroxine dose > 200 mcg

Consider pituitary function and cortisol if secondary hypothyroidism suspected (TSH low, FT4 low).

 

What is Hyperthyroidism?

Hyperthyroidism is a condition of excess thyroid hormone production, leading to suppressed TSH and raised free T4 and/or T3. It can cause a wide range of symptoms including palpitations, weight loss, heat intolerance, and anxiety.

Stepwise Clinical Approach

Step 1: Initial TSH Screening

If TSH is < 0.4 mU/L and patient is not on thyroxine:

  • TSH 0.1 - 0.4 mU/L: repeat TSH, FT4 and FT3 in 1 - 2 months
  • TSH < 0.1 mU/L: check FT4 and FT3 immediately

Step 2: Investigate and Exclude Other Causes

  • Review for non-thyroidal illness or drug causes (e.g. steroids, dopamine, amiodarone)
  • If not treated, repeat TSH every 6 - 12 months with FT3/FT4 if TSH drops further

Step 3: Management of Confirmed Hyperthyroidism

  • Refer to endocrinology if TSH < 0.1 mU/L with abnormal FT4 or FT3
  • If symptomatic, consider propranolol 10 - 40 mg tds for symptom relief
  • Carbimazole may be initiated in consultation with endocrinology (warn about rash, agranulocytosis)

Step 4: Ongoing Management

  • Definitive treatment may include anti-thyroid drugs, radio-iodine, or surgery
  • Check TFT every 1 - 3 months on treatment until stable
  • Once stable and on long-term treatment, check TFT yearly

Step 5: Red Flags Requiring Urgent Review

  • Very low TSH (< 0.01) with high FT3/FT4 and clinical signs of thyrotoxicosis
  • New onset atrial fibrillation
  • Severe weight loss, tremor or eye signs (Graves’)
  • Pregnancy or planning pregnancy - refer urgently
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