Why would a 34-year-old patient with primary hypothyroidism, characterized by a Thyroid-Stimulating Hormone (TSH) level of 5 and symptoms of feeling cold, be advised to hold off on levothyroxine (T4) therapy and instead undergo a thyroid ultrasound?

Why Another Provider Ordered a Thyroid Ultrasound Instead of Starting Levothyroxine

The other provider likely ordered a thyroid ultrasound to rule out structural thyroid abnormalities (nodules, goiter, or malignancy) before committing to lifelong levothyroxine therapy, though this approach contradicts current guidelines—imaging has no role in the routine workup of primary hypothyroidism in adults. 1

The Standard Approach: Levothyroxine Should Be Started

Your initial management was appropriate. For a 34-year-old patient with TSH of 5 mIU/L and hypothyroid symptoms (feeling cold), levothyroxine therapy is indicated according to established guidelines 2:

• TSH persistently >4.5 mIU/L with symptoms warrants treatment, even though this patient's TSH is below the 10 mIU/L threshold where treatment becomes mandatory regardless of symptoms 2
• Symptomatic patients with any degree of TSH elevation should receive thyroid hormone supplementation 3, 2
• The starting dose of 88 mcg is reasonable for a young adult without cardiac disease, approximating the 1.6 mcg/kg/day full replacement dose 2, 4

Why Imaging Is Not Indicated

The American College of Radiology explicitly states there is no role for thyroid ultrasound, CT, MRI, or radionuclide scanning in the workup of primary hypothyroidism in adults 1:

• Imaging does not differentiate among causes of hypothyroidism 1
• All causes of hypothyroidism show decreased radioiodine uptake, making functional imaging unhelpful 1
• The diagnosis of primary hypothyroidism is biochemical, not anatomical 4

Possible Rationale for the Ultrasound Order

The other provider may have been concerned about:

1. Ruling Out Thyroid Nodules or Malignancy

• While palpable thyroid abnormalities or concerning features would warrant imaging, there is no indication this patient has a palpable nodule or goiter 1
• Thyroid cancer does not typically present with hypothyroidism and elevated TSH 1
• If a nodule were palpable, ultrasound would be appropriate, but this should not delay levothyroxine initiation 1

2. Confirming Hashimoto's Thyroiditis

• Some providers order ultrasound to look for the heterogeneous echotexture characteristic of chronic autoimmune thyroiditis 1
• However, this is unnecessary—anti-TPO antibody testing is the appropriate way to confirm autoimmune etiology, not imaging 2, 1
• The presence of anti-TPO antibodies predicts 4.3% annual progression to overt hypothyroidism versus 2.6% in antibody-negative patients 2

3. Assessing Goiter Size

• If the patient has a visibly enlarged thyroid, ultrasound can quantify goiter size 1
• But this does not change initial management—levothyroxine is still the treatment 1, 5

What Should Actually Be Done Before Starting Levothyroxine

Instead of ordering imaging, the appropriate pre-treatment workup includes:

Confirm the Diagnosis

• Repeat TSH with free T4 after 3-6 weeks, as 30-60% of elevated TSH levels normalize spontaneously 2
• Measure both TSH and free T4 to distinguish subclinical hypothyroidism (normal free T4) from overt hypothyroidism (low free T4) 2

Identify the Etiology

• Check anti-TPO antibodies to confirm autoimmune thyroiditis (Hashimoto's disease), the most common cause in iodine-sufficient areas 2, 1, 4
• This predicts progression risk and justifies treatment 2

Rule Out Central Hypothyroidism

• If TSH is low or inappropriately normal with low free T4, this suggests pituitary/hypothalamic disease rather than primary thyroid failure 3
• In central hypothyroidism, adrenal insufficiency must be ruled out before starting levothyroxine, as thyroid hormone can precipitate adrenal crisis 3, 2, 5

Critical Safety Consideration

Before initiating or increasing levothyroxine, always rule out concurrent adrenal insufficiency, especially in suspected central hypothyroidism 2, 5:

• If cortisol is low, thyroid hormone increases cortisol metabolism and can trigger life-threatening adrenal crisis 3
• Start hydrocortisone first when multiple pituitary hormones are deficient 3
• However, with TSH of 5 mIU/L, this patient clearly has primary hypothyroidism, not central hypothyroidism 2

The Correct Management Algorithm

For this 34-year-old symptomatic patient with TSH 5 mIU/L:

1. Confirm diagnosis: Repeat TSH and measure free T4 after 3-6 weeks 2
2. Check anti-TPO antibodies to confirm autoimmune etiology 2, 1
3. Start levothyroxine 88 mcg daily (your original plan was correct) 2, 4
4. Monitor TSH every 6-8 weeks while titrating to target TSH 0.5-4.5 mIU/L 2
5. Once stable, monitor TSH every 6-12 months 2

Common Pitfall to Avoid

Do not delay treatment while waiting for imaging results 2, 1:

• Untreated hypothyroidism causes cardiovascular dysfunction, adverse lipid profiles, and decreased quality of life 2
• The median TSH at which treatment is initiated has decreased from 8.7 to 7.9 mIU/L in recent years, supporting treatment at TSH 5 mIU/L in a symptomatic patient 2
• Approximately 25% of patients are inadvertently undertreated due to excessive caution, leading to persistent symptoms 2

The thyroid ultrasound order represents a deviation from evidence-based guidelines and unnecessarily delays appropriate treatment for this symptomatic patient with biochemically confirmed hypothyroidism.