I have a thyroid‑stimulating hormone of 5.8 mIU/L and a free thyroxine of 0.75 µg/dL; what diagnosis does this suggest and how should it be managed?

Diagnosis: Overt Hypothyroidism

Your laboratory values—TSH 5.8 mIU/L with free T4 0.75 µg/dL (which is below the normal range of approximately 0.8–1.8 µg/dL)—indicate overt hypothyroidism requiring immediate levothyroxine therapy. This is not subclinical hypothyroidism; the combination of elevated TSH and low free T4 defines overt disease that mandates treatment 1.

Confirming the Diagnosis

• Repeat TSH and free T4 in 3–6 weeks to confirm persistent elevation, as 30–60% of mildly abnormal TSH values normalize spontaneously 1, 2.
• However, given that your free T4 is frankly low (not just borderline), treatment should not be delayed while awaiting confirmatory testing if you are symptomatic 1.
• Measure anti-thyroid peroxidase (anti-TPO) antibodies to identify autoimmune thyroiditis (Hashimoto's disease), which predicts a 4.3% annual progression risk versus 2.6% in antibody-negative individuals 1.

Why Immediate Treatment Is Required

Overt hypothyroidism (elevated TSH with low free T4) requires prompt levothyroxine initiation to prevent cardiovascular dysfunction, adverse lipid profiles, and quality of life deterioration 1. Your TSH of 5.8 mIU/L combined with a subnormal free T4 places you at risk for:

• Cardiac dysfunction including delayed myocardial relaxation, reduced cardiac output, and increased systemic vascular resistance 3, 1.
• Adverse lipid changes with elevated total cholesterol and LDL cholesterol 3, 1.
• Progressive symptoms including fatigue, weight gain, cold intolerance, constipation, cognitive slowing, and menstrual irregularities 1.

Levothyroxine Dosing Strategy

Initial Dose Selection

• For patients <70 years without cardiac disease or multiple comorbidities, start levothyroxine at approximately 1.6 mcg/kg/day (typically 100–125 mcg daily for most adults) 1.
• For patients >70 years or with cardiac disease/multiple comorbidities, start with a lower dose of 25–50 mcg/day and titrate gradually by 12.5–25 mcg every 6–8 weeks to avoid unmasking cardiac ischemia or precipitating arrhythmias 1.

Critical Safety Precaution

Before initiating levothyroxine, rule out concurrent adrenal insufficiency by checking morning cortisol and ACTH, especially if you have unexplained hypotension, hyponatremia, or hyperpigmentation 1. Starting thyroid hormone before adequate corticosteroid coverage can precipitate life-threatening adrenal crisis 1.

Monitoring Protocol

• Recheck TSH and free T4 in 6–8 weeks after starting levothyroxine, as this represents the time needed to reach steady state 1.
• Target TSH range is 0.5–4.5 mIU/L with normal free T4 levels 1.
• Continue dose adjustments by 12.5–25 mcg increments every 6–8 weeks until TSH normalizes 1.
• Once stable, repeat testing every 6–12 months or sooner if symptoms change 1.

Common Pitfalls to Avoid

• Do not treat based on a single TSH value alone—always measure free T4 to distinguish subclinical from overt hypothyroidism 1.
• Never start thyroid hormone before ruling out adrenal insufficiency in suspected central hypothyroidism, as this can precipitate adrenal crisis 1.
• Avoid overtreatment, which occurs in 14–21% of treated patients and increases risk for atrial fibrillation, osteoporosis, fractures, and cardiac complications 1.
• Do not adjust doses more frequently than every 6–8 weeks, as levothyroxine requires this interval to reach steady state 1.

Special Populations Requiring Modified Approach

If You Are Pregnant or Planning Pregnancy

• Treat immediately with levothyroxine, targeting TSH <2.5 mIU/L in the first trimester 1.
• Untreated maternal hypothyroidism increases risk of preeclampsia, low birth weight, and permanent neurodevelopmental deficits in the child 1.
• Levothyroxine requirements typically increase by 25–50% during pregnancy 1.

If You Have Cardiac Disease

• Start at 25–50 mcg/day regardless of age to avoid precipitating myocardial infarction, heart failure, or arrhythmias 1.
• Obtain an ECG to screen for baseline arrhythmias 1.
• Monitor closely for angina, palpitations, dyspnea, or worsening heart failure 1.

Expected Outcomes with Treatment

• Symptom improvement typically occurs within 6–8 weeks of achieving target TSH 1.
• Cardiovascular function improves, with normalization of cardiac output and reduction in systemic vascular resistance 1.
• Lipid profile improves, with significant reductions in total cholesterol, LDL cholesterol, and triglycerides 1.
• Quality of life improves through resolution of fatigue, cognitive impairment, and other hypothyroid symptoms 1.