What laboratory tests should be monitored in a patient with hypothyroidism treated with pork thyroid (desiccated thyroid), such as Armour Thyroid (thyroxine (T4) and triiodothyronine (T3))?

Laboratory Monitoring for Pork Thyroid (Desiccated Thyroid Extract)

Primary Monitoring Parameters

Monitor TSH and free T4 every 6-8 weeks during dose titration, then every 6-12 months once stable. 1, 2 This is the cornerstone of monitoring for any thyroid hormone replacement, including desiccated thyroid products like Armour Thyroid.

Initial Monitoring Phase

• Check TSH and free T4 at 6-8 week intervals after starting therapy or any dose adjustment, as this represents the time needed to reach steady state 1, 2
• Measure free T3 levels in addition to TSH and free T4, particularly important with desiccated thyroid since it contains both T4 and T3 3, 4
• The combination of T4 and T3 in desiccated thyroid (approximately 4:1 ratio) means you must monitor both hormones to avoid T3 toxicity 4

Why Free T3 Monitoring is Critical with Desiccated Thyroid

Unlike levothyroxine monotherapy where T3 monitoring is generally unnecessary, desiccated thyroid contains preformed T3 that can cause transient hypertriiodothyroninemia 4, 5. This is the key difference in monitoring:

• Free T3 levels may rise above the reference range during the absorption phase after taking desiccated thyroid, potentially causing palpitations and other hyperthyroid symptoms 5
• The mean daily dose of desiccated thyroid needed to normalize TSH contains approximately 11 mcg of T3, but some patients require higher doses 4
• Monitor for symptoms of T3 excess: rapid or irregular heartbeat, chest pain, tremors, nervousness, heat intolerance 2

Maintenance Monitoring

Once the appropriate dose is established and TSH is normalized:

• Annual TSH and free T4 measurement is sufficient for stable patients 1, 2
• Continue monitoring free T3 at least annually given the T3 content of desiccated thyroid 3, 4
• Recheck sooner if symptoms change or new medications are started 1, 2

Additional Laboratory Tests

Before Starting Therapy

• Morning cortisol and ACTH to rule out adrenal insufficiency, as starting thyroid hormone before addressing adrenal insufficiency can precipitate life-threatening adrenal crisis 6, 1
• Baseline glucose and HbA1c to monitor glycemic control, as thyroid hormone can affect insulin requirements 6, 2
• Anti-TPO antibodies if autoimmune thyroiditis is suspected, as this predicts higher progression risk (4.3% vs 2.6% per year) 1, 7

Ongoing Monitoring

• Fasting lipid panel can be checked periodically, as hypothyroidism affects cholesterol metabolism and treatment should improve lipid profiles 1, 8
• Complete metabolic panel before each cycle initially to monitor electrolytes and glucose trends 6
• Prothrombin time/INR if patient is on anticoagulants, as thyroid hormone increases sensitivity to warfarin 3
• Blood glucose monitoring in diabetic patients, as insulin or oral hypoglycemic requirements may increase 3

Critical Safety Monitoring

Cardiovascular Monitoring

• Obtain baseline ECG in patients over 60 years or with known cardiac disease before starting desiccated thyroid 1
• Monitor for atrial fibrillation, especially if TSH becomes suppressed (<0.1 mIU/L), which increases AF risk 3-5 fold 1
• Assess for angina, palpitations, or dyspnea at each follow-up, particularly during dose titration 1, 2

Bone Health Monitoring

• Consider bone density assessment in postmenopausal women if TSH remains chronically suppressed, as this accelerates bone loss 1
• Ensure adequate calcium (1200 mg/day) and vitamin D (1000 units/day) intake in patients with suppressed TSH 1

Target Laboratory Values

• TSH: 0.5-4.5 mIU/L for most patients with primary hypothyroidism 1, 2
• Free T4: upper half of normal range is typical with thyroid hormone replacement 5
• Free T3: within normal reference range, avoiding supranormal peaks that can occur with desiccated thyroid 4, 5
• Avoid TSH suppression below 0.1 mIU/L unless treating thyroid cancer, as this significantly increases cardiovascular and bone risks 1

Special Populations

Pregnancy

• Check TSH and free T4 every 4 weeks until stable, then at minimum once per trimester 1
• Target TSH <2.5 mIU/L in first trimester 1
• Note: Levothyroxine monotherapy is strongly preferred over desiccated thyroid during pregnancy, as T3 supplementation provides inadequate fetal thyroid hormone delivery 1

Elderly or Cardiac Patients

• More frequent monitoring may be warranted, consider repeating tests within 2 weeks of dose adjustment rather than waiting 6-8 weeks 1
• Start with lower doses (12.5-25 mcg/day) and titrate slowly to avoid cardiac complications 1, 2

Common Pitfalls to Avoid

• Never adjust doses more frequently than every 6-8 weeks before steady state is reached 1
• Do not ignore suppressed TSH even if patient feels well, as this carries significant cardiovascular and bone risks 1
• Approximately 25% of patients on thyroid replacement are unintentionally overtreated with fully suppressed TSH, increasing risks for atrial fibrillation, osteoporosis, and fractures 1, 8
• Missing the T3 peaks that occur with desiccated thyroid by only checking TSH and T4 4, 5
• Failing to check morning cortisol before starting therapy in patients with suspected central hypothyroidism or multiple pituitary deficiencies 6, 1

Timing of Blood Draw

• Take blood sample before ingesting the daily dose of desiccated thyroid to avoid measuring the absorption peak of T3 5
• Morning fasting sample is preferred for consistency 6
• Wait at least 4 hours after taking desiccated thyroid if morning dose already taken, though pre-dose sampling is ideal 2