What investigations should be done in a patient with newly detected hypothyroidism?

Investigations for Newly Detected Hypothyroidism

The essential investigations for newly detected hypothyroidism should include thyroid function tests (TSH, free T4), thyroid antibodies, and assessment of other pituitary hormones if central hypothyroidism is suspected. 1, 2

Initial Diagnostic Workup

• Thyroid Function Tests: Confirm the diagnosis with TSH and free T4 measurements. In primary hypothyroidism, TSH will be elevated with low/normal free T4, while in central hypothyroidism, both TSH and free T4 will be low 1, 3
• Thyroid Antibodies: Test for thyroid peroxidase (TPO) antibodies to identify autoimmune etiology (Hashimoto's thyroiditis), which is the most common cause of primary hypothyroidism in iodine-sufficient areas 2, 4
• Morning Cortisol and ACTH: These tests are particularly important if central hypothyroidism is suspected, as adrenal insufficiency must be ruled out before starting thyroid hormone replacement 1, 5
• Lipid Profile: Hypothyroidism can cause dyslipidemia, so baseline lipid levels should be assessed 1, 2
• Complete Blood Count: To evaluate for anemia, which can be associated with hypothyroidism 2
• Metabolic Panel: To assess baseline glycemic status, as hypothyroidism can affect glucose metabolism 1, 2

Additional Investigations Based on Clinical Suspicion

• Pituitary Function Tests: If central hypothyroidism is suspected (low TSH with low free T4), evaluate other pituitary hormones including gonadal hormones (FSH, LH, testosterone in men, estradiol in women) 1, 5
• Pituitary MRI: Indicated in cases of central hypothyroidism to evaluate for pituitary abnormalities 1, 5
• Pregnancy Test: In women of reproductive age, as management differs in pregnancy 4
• Thyroid Ultrasound: Consider if thyroid gland enlargement or nodules are detected on physical examination 4

Special Considerations

• Central vs. Primary Hypothyroidism: Distinguishing between these is crucial as management differs. Central hypothyroidism presents with low/normal TSH and low free T4, while primary hypothyroidism shows elevated TSH and low free T4 5, 3
• Subclinical Hypothyroidism: When TSH is elevated but free T4 is normal, repeat testing in 2-3 months to confirm persistence before initiating treatment 1
• Thyroiditis: In cases where thyrotoxicosis precedes hypothyroidism, additional testing may include thyroid stimulating hormone receptor antibody (TRAb) or thyroid stimulating immunoglobulin (TSI) to distinguish between thyroiditis and Graves' disease 1

Monitoring After Diagnosis

• TSH and Free T4: Recheck 6-8 weeks after starting treatment or changing dose 2
• In Primary Hypothyroidism: Monitor TSH with target of 0.5-2.0 mIU/L 4
• In Central Hypothyroidism: Monitor free T4 levels, maintaining them in the upper half of the normal range 5, 4

Common Pitfalls to Avoid

• Relying solely on physical examination: Physical signs have limited diagnostic accuracy for hypothyroidism (positive likelihood ratios range from 1.0 to 3.88), making laboratory confirmation essential 6
• Overlooking central hypothyroidism: Always consider this diagnosis when TSH is low/normal with low free T4 5
• Starting thyroid replacement before ruling out adrenal insufficiency: In patients with central hypothyroidism, hydrocortisone should be started before thyroid hormone to avoid precipitating an adrenal crisis 1
• Overdiagnosis: Be cautious about labeling patients with mild TSH elevations as hypothyroid, as approximately 37% of patients with initially elevated TSH may spontaneously revert to normal thyroid function 1