No, This is NOT Subclinical Hypothyroidism—This is Overt Hypothyroidism
A patient with elevated TSH AND low T4 levels has overt hypothyroidism, not subclinical hypothyroidism, regardless of symptom severity. 1
Clear Diagnostic Distinction
The terminology is based strictly on laboratory values, not symptoms:
- Subclinical hypothyroidism is defined as elevated TSH (>4.5 mIU/L) with normal free T4 levels 1
- Overt hypothyroidism is defined as elevated TSH with low free T4 levels 1, 2
Your patient has overt hypothyroidism because the T4 is low. The presence or absence of symptoms does not change this classification. 1, 2
Why This Distinction Matters Clinically
The difference between subclinical and overt hypothyroidism has major treatment implications:
- Overt hypothyroidism requires treatment in all cases—there is no controversy or need for individualized decision-making 3, 2
- Treatment should be initiated promptly to prevent serious complications including heart failure, cardiovascular events, and progression to myxedema coma (which carries up to 30% mortality) 2
- Untreated overt hypothyroidism causes insulin resistance, hyperglycemia in diabetic patients, menstrual irregularities, infertility, and increased miscarriage risk 2
In contrast, subclinical hypothyroidism (normal T4) generates debate about treatment thresholds, particularly when TSH is between 4.5-10 mIU/L. 1, 4
Treatment Algorithm for Your Patient
Since this is overt hypothyroidism, initiate levothyroxine immediately:
- For patients <70 years without cardiac disease: Start with full replacement dose of approximately 1.6 mcg/kg/day 4, 3
- For patients >70 years or with coronary artery disease: Start with 25-50 mcg/day and titrate gradually to avoid cardiac complications 4, 5, 3
- For patients with long-standing severe hypothyroidism: Use lower starting doses regardless of age 3
Monitor TSH and free T4 in 6-8 weeks after initiating therapy, targeting TSH of 0.5-4.5 mIU/L. 4, 3, 2
Critical Safety Consideration
Before starting levothyroxine, rule out concurrent adrenal insufficiency, especially in patients with autoimmune hypothyroidism or suspected central hypothyroidism. Starting thyroid hormone before corticosteroids can precipitate life-threatening adrenal crisis. 4, 3
Common Pitfall to Avoid
Do not delay treatment waiting for symptom severity to worsen. The biochemical definition (low T4) mandates treatment regardless of how symptomatic the patient appears. 3, 2 Even patients who seem minimally symptomatic may have subtle cardiac dysfunction, lipid abnormalities, and metabolic derangements that improve with treatment. 1, 2