What is the diagnostic plan for hypothyroidism?

Diagnostic Plan for Hypothyroidism

The diagnostic approach for hypothyroidism should begin with measuring TSH, followed by free T4 if TSH is elevated, with repeat testing in 2-3 months to confirm abnormal results before initiating treatment. 1, 2

Initial Diagnostic Testing

• Measure TSH as the first-line test for suspected hypothyroidism 1, 3
• If TSH is elevated, measure free T4 to differentiate between subclinical hypothyroidism (normal free T4) and overt hypothyroidism (low free T4) 1, 2
• Repeat abnormal results in 2 weeks to 3 months before initiating treatment to confirm diagnosis and avoid overdiagnosis 1, 2
• Perform morning testing for more accurate results, especially when assessing adrenal function simultaneously 1, 2

Interpretation of Results

• Primary overt hypothyroidism: Elevated TSH with low free T4 4
• Subclinical hypothyroidism: Elevated TSH (4.5-10 mIU/L) with normal free T4 1, 2
• Central hypothyroidism: Low or inappropriately normal TSH with low free T4 (due to pituitary or hypothalamic dysfunction) 2

Additional Testing to Consider

• Anti-thyroid peroxidase (anti-TPO) antibodies to identify autoimmune etiology (Hashimoto's thyroiditis), which predicts a higher risk of developing overt hypothyroidism (4.3% per year vs. 2.6% per year in antibody-negative individuals) 1, 2
• Lipid profile testing, as hypothyroidism can affect cholesterol levels, with significant increases in LDL cholesterol and triglycerides 1, 2

Special Populations Requiring Targeted Testing

• Patients with type 1 diabetes (10-20% prevalence of hypothyroidism) 1
• Pregnant women or those planning pregnancy 2
• Patients with family history of thyroid disease 4
• Patients who have undergone neck surgery or radiation therapy 4
• Patients taking medications that can cause hypothyroidism (e.g., amiodarone, immune checkpoint inhibitors) 4

Common Pitfalls to Avoid

• Relying solely on physical examination: Physical signs have poor diagnostic accuracy for hypothyroidism (positive likelihood ratios range from 1.0 to 3.88), making laboratory testing essential 5
• Misdiagnosis of subclinical hypothyroidism: Not all patients with TSH between 4.5-10 mIU/L require treatment, especially if asymptomatic 1, 2
• Euthyroid sick syndrome: Thyroid function tests may be misleading if performed during acute illness, ketosis, or significant weight loss 1
• Overdiagnosis: Many persons labeled with hypothyroidism spontaneously revert to a euthyroid state over time; others never progress to noticeable health problems 6
• Initiating thyroid hormone replacement before ruling out adrenal insufficiency: This can lead to adrenal crisis 2

No Role for Routine Imaging

• Thyroid ultrasound, CT, MRI, or radionuclide scans are not indicated for the initial workup of hypothyroidism 1

Clinical Context

• Hypothyroidism affects up to 5% of the general population, with another 5% being undiagnosed 7
• Common symptoms include fatigue (68-83%), weight gain (24-59%), cognitive issues (45-48%), and menstrual irregularities (23%) 4
• Hashimoto's thyroiditis is the cause of primary hypothyroidism in up to 85% of patients in iodine-sufficient areas 4
• Untreated hypothyroidism can progress to severe complications including heart failure and myxedema coma (with mortality rates up to 30%) 4, 8