Approach to Managing Hypothyroidism

Initial Diagnosis and Confirmation

The American College of Endocrinology recommends measuring TSH as the primary screening test for suspected hypothyroidism, and if elevated, confirming with repeat testing after 3-6 weeks along with free T4 measurement, as 30-60% of elevated TSH levels normalize spontaneously 1
TSH is the most sensitive test for detecting hypothyroidism with sensitivity above 98% and specificity greater than 92% 1
Measuring both TSH and free T4 can distinguish between subclinical hypothyroidism (elevated TSH with normal free T4) and overt hypothyroidism (elevated TSH with low free T4) 1
Consider measuring anti-TPO antibodies to identify autoimmune etiology, which predicts higher progression risk to overt hypothyroidism (4.3% per year vs 2.6% in antibody-negative individuals) 1

If low free T4 is found with normal or low TSH, suspect central hypothyroidism from pituitary or hypothalamic dysfunction 2
Before initiating thyroid hormone replacement in suspected central hypothyroidism, always evaluate for concurrent adrenal insufficiency first, as starting levothyroxine before corticosteroids can precipitate life-threatening adrenal crisis 3, 2

Initiate levothyroxine therapy immediately for all patients with TSH >10 mIU/L, as this threshold carries approximately 5% annual risk of progression to overt hypothyroidism and may prevent cardiovascular complications 1
For TSH between 4.5-10 mIU/L with normal free T4, routine levothyroxine treatment is NOT recommended; instead monitor thyroid function every 6-12 months 1
Consider treatment in specific situations, such as pregnant women or those planning pregnancy, positive anti-TPO antibodies, or symptomatic patients 1

Start with full replacement dose of approximately 1.6 mcg/kg/day for patients <70 years without cardiac disease 1
Start with lower dose of 25-50 mcg/day and titrate gradually for patients >70 years OR with cardiac disease/multiple comorbidities 1

For patients on immune checkpoint inhibitors, thyroid dysfunction occurs in 6-20% of patients, and monitor TSH every 4-6 weeks for first 3 months, then every second cycle 3, 1
In presence of both adrenal insufficiency and hypothyroidism, always start steroids before thyroid hormone to avoid adrenal crisis 3

For confirmed central hypothyroidism, rule out and treat adrenal insufficiency FIRST, and start hydrocortisone before levothyroxine 3, 2
Evaluate for other pituitary hormone deficiencies (approximately 50% have panhypopituitarism) 3, 2
Start levothyroxine and monitor free T4 levels (NOT TSH) 2

When properly dosed, levothyroxine is safe for lifelong use, but overtreatment carries significant risks, including atrial fibrillation, osteoporosis, and cardiovascular mortality 1
Critical monitoring points include development of low TSH on therapy, and avoiding adjusting doses more frequently than every 6-8 weeks before reaching steady state 1