Thyroid Function Testing and Management

Diagnosis and Testing

The American College of Endocrinology recommends checking for hypothyroidism (underactive thyroid) with low free T4 and high TSH in primary thyroid failure 1
The Endocrine Society suggests that blood will be drawn from the arm, typically in the morning around 8 AM for most accurate results 2
No special preparation is usually required unless the doctor specifies otherwise 2

If free T4 is low with high TSH, it indicates primary hypothyroidism, where the thyroid gland itself is not working properly 1
If free T4 is low with low/normal TSH, it may indicate central hypothyroidism, requiring additional testing including morning cortisol levels and pituitary MRI 1
Elevated total T3 with normal free T4 may represent early hyperthyroidism, subacute thyroiditis, Graves' disease, or toxic nodular goiter, and the doctor will likely recheck these tests every 2-3 weeks to monitor for changes 3

The American Thyroid Association recommends that if you're already taking levothyroxine (thyroid hormone replacement), free T4 is the most important test to monitor your treatment 1
If you have suspected central hypothyroidism, never start thyroid hormone replacement before ruling out adrenal insufficiency, as this could trigger a life-threatening adrenal crisis 1
The Endocrine Society suggests monitoring every 6-12 months once the condition is stable 1

During initial diagnosis or dose adjustments, recheck every 2-3 weeks for unstable conditions or every 6-8 weeks for stable dose changes 1, 3
Seek immediate medical attention if you experience severe symptoms of hyperthyroidism, symptoms of adrenal crisis, or worsening fatigue, confusion, or cold intolerance suggesting severe hypothyroidism 1, 3

For diagnostic evaluation of adrenal insufficiency or hypophysitis, morning ACTH and cortisol levels should be obtained preferably around 8 AM, but no fasting requirement is specified 4, 5
Patients should avoid heavy meals within 2-3 hours before testing and abstain from caffeine on the day of testing, but complete fasting is not required 6

When evaluating suspected central hypothyroidism, always check morning cortisol levels simultaneously to rule out adrenal insufficiency before initiating thyroid hormone replacement 4, 5
In hypophysitis evaluation, obtain TSH, free T4, ACTH, and cortisol together, preferably in the morning around 8 AM 4, 5

Schedule cortisol testing between 7-9 AM for accurate baseline values 4, 5
Allow light meals or snacks before testing, avoiding only heavy meals within 2-3 hours 6
Avoid caffeine on the day of testing 6