Evaluation for Hypothyroidism
Begin with serum TSH and free T4 measurement, preferably obtained in the morning around 8 am, as these are the cornerstone tests for diagnosing hypothyroidism. 1, 2
Clinical Presentation
Suspect hypothyroidism in patients presenting with:
- Fatigue (68-83% of cases), weight gain (24-59%), cold intolerance, constipation, and depression 3, 2
- Hair loss, dry skin, voice changes, and menstrual irregularities (approximately 23% experience oligomenorrhea or menorrhagia) 3, 1, 2
- More advanced symptoms including intellectual slowness, insomnia, and in severe cases, myxedema 1
- Note: Physical examination alone cannot reliably confirm or exclude hypothyroidism—laboratory testing is mandatory 4
Diagnostic Algorithm
Initial Laboratory Testing
Order TSH and free T4 as the primary diagnostic tests:
- High TSH with low free T4 = overt primary hypothyroidism 2, 5
- High TSH with normal free T4 = subclinical hypothyroidism 5
- Low/normal TSH with low free T4 = central (secondary) hypothyroidism—requires different evaluation 3, 1
Additional Testing When Hypothyroidism Confirmed
Once biochemical hypothyroidism is established, obtain thyroid peroxidase (TPO) antibodies to identify autoimmune etiology (Hashimoto thyroiditis causes up to 85% of cases in iodine-sufficient areas) 3, 2
Critical Consideration for Central Hypothyroidism
If central hypothyroidism is suspected (low TSH with low free T4), you must evaluate for hypophysitis and other pituitary hormone deficiencies BEFORE starting treatment: 3, 1
- Obtain ACTH, cortisol (or 1 mcg cosyntropin stimulation test) 3
- Check gonadal hormones (testosterone in men, estradiol in women), FSH, and LH 3
- Order MRI of the sella with pituitary cuts 3
- These tests should be performed prior to steroid administration 3
Treatment Initiation
Primary Hypothyroidism Treatment
Start levothyroxine (LT4) as first-line therapy: 2, 5
- Standard dosing: 1.6 mcg/kg/day for most adults 6
- Lower starting doses required for: 6, 5
- Elderly patients
- Patients with coronary artery disease or atrial fibrillation
- Patients with long-standing severe hypothyroidism
- Young, otherwise healthy patients can often start with full calculated dose 5
Critical Safety Rule
When both adrenal insufficiency and hypothyroidism coexist, ALWAYS start steroids before thyroid hormone replacement to prevent precipitating an adrenal crisis 3, 1
Monitoring Strategy
Initial Monitoring
Check TSH 6-8 weeks after starting treatment or after any dose adjustment 6, 2
- Target TSH: 0.5-2.0 mIU/L for primary hypothyroidism 5
- For central hypothyroidism: monitor free T4 levels instead, maintaining them in the upper half of normal range 6, 5
Long-term Monitoring
Once TSH is stable, recheck every 6-12 months and whenever clinical status changes 6, 2
Screening for Associated Conditions
Annual screening is essential because hypothyroidism frequently coexists with other autoimmune conditions: 3
- TSH, free T4, and TPO antibodies every 12 months to detect thyroid dysfunction progression 3
- Fasting glucose and HbA1c for diabetes mellitus 3
- Complete blood count for anemia 3
- Vitamin B12 levels for pernicious anemia 3
- Tissue transglutaminase antibodies and total IgA for celiac disease if gastrointestinal symptoms present 3
Common Pitfalls to Avoid
Persistently elevated TSH despite adequate LT4 dosing suggests: 6, 5
- Poor medication compliance (most common)
- Malabsorption issues
- Drug interactions
- Inadequate dosing
Over-replacement is common and dangerous—associated with increased risk of atrial fibrillation and osteoporosis 5
Special Populations
Pregnant Patients
For pregnant patients with pre-existing hypothyroidism: 6
- Measure TSH and free T4 immediately upon pregnancy confirmation
- Increase levothyroxine dose by 12.5-25 mcg/day to maintain TSH in trimester-specific range 6
- Monitor TSH every 4 weeks until stable 6
- Return to pre-pregnancy dose immediately after delivery 6
Subclinical Hypothyroidism Treatment Decisions
Treat subclinical hypothyroidism when: 5
- TSH >10 mIU/L (treat all patients)
- Any TSH elevation in pregnant women or women contemplating pregnancy
- TSH ≤10 mIU/L with symptoms, infertility, goiter, or positive TPO antibodies
Consider avoiding treatment in patients >85 years with TSH ≤10 mIU/L 5