Free T4 (Thyroxine) Level
In a patient with TSH >300 mIU/L, you must immediately measure free T4 to confirm the diagnosis of overt primary hypothyroidism and assess its severity. This is the essential next step before initiating treatment.
Diagnostic Algorithm
When TSH is abnormal, the diagnostic approach follows a clear hierarchy 1, 2, 3:
Measure free T4 - This distinguishes between:
- Overt hypothyroidism: Elevated TSH + Low free T4 (most likely with TSH >300)
- Subclinical hypothyroidism: Elevated TSH + Normal free T4 (unlikely with TSH this high)
The TSH-free T4 relationship provides >90% diagnostic accuracy for thyroid disorders 1. With a TSH >300 mIU/L, you're almost certainly dealing with severe overt primary hypothyroidism, but the free T4 level quantifies the degree of thyroid hormone deficiency and guides initial levothyroxine dosing.
Why Free T4 is Critical
TSH alone is never adequate to initiate treatment 3. The diagnosis must be based on the relationship between the trophic hormone (TSH) and target gland hormone (free T4).
A TSH >300 mIU/L is extraordinarily elevated—far beyond the typical reference range upper limit of ~4-5 mIU/L 4, 1. This magnitude virtually excludes the confounding factors that affect TSH interpretation at lower elevations (recovery from illness, medications, age-related changes).
Important Caveats
While less likely with such extreme TSH elevation, be aware of:
Assay interference: Heterophilic antibodies, anti-TSH antibodies, or macro-TSH can cause falsely elevated TSH values 4, 5, 6. However, these typically produce TSH elevations in the range of 6-50 mIU/L, not >300.
Central hypothyroidism with TSH-secreting adenoma: Would show elevated TSH with elevated (not low) free T4 5—the opposite pattern expected here.
Additional Considerations
After confirming the diagnosis with free T4, consider:
- Anti-thyroid peroxidase (TPO) antibodies to identify autoimmune thyroid disease as the etiology
- Clinical assessment for myxedema coma if the patient is critically ill, hypothermic, or has altered mental status
- Baseline lipid panel and CBC before starting treatment
The free T4 result will confirm severe hypothyroidism and allow you to initiate appropriate levothyroxine replacement therapy immediately.