Free Thyroxine

When low (hypothyroid tendency):

Supplements:

• Selenium -- 200mcg/day (selenomethionine), supports thyroid peroxidase and T4 synthesis
• Iodine -- 150-300mcg/day (kelp or potassium iodide), essential substrate for T4; do NOT mega-dose
• L-Tyrosine -- 500-1000mg/day (morning, empty stomach), amino acid precursor to thyroid hormones
• Zinc -- 30mg/day (picolinate), cofactor for thyroid hormone synthesis
• Ashwagandha -- 300-600mg/day, may support thyroid output in subclinical hypothyroidism

Lifestyle:

• Ensure adequate caloric intake -- prolonged deficits suppress T4 production
• Address chronic stress (elevated cortisol inhibits TSH secretion and T4 output)
• If on exogenous T3, low T4 is expected (negative feedback suppresses TSH and endogenous T4); taper T3 slowly over 2-3 weeks to allow thyroid axis recovery

When high (without exogenous thyroid hormones):

• Recheck with TSH and Free T3 to confirm hyperthyroidism
• If TSH is suppressed with elevated Free T4, investigate Graves' disease or thyroiditis
• Reduce or eliminate iodine supplementation if excessive
• Refer to endocrinologist if confirmed

References:

• Garber, J. R., Cobin, R. H., Gharib, H., et al. (2012). Clinical practice guidelines for hypothyroidism in adults. Thyroid, 22(12), 1200-1235. DOI: 10.1089/thy.2012.0205
• Ventura, M., Melo, M., & Carrilho, F. (2017). Selenium and thyroid disease: From pathophysiology to treatment. International Journal of Endocrinology, 2017, 1297658. DOI: 10.1155/2017/1297658
• Zimmermann, M. B., & Boelaert, K. (2015). Iodine deficiency and thyroid disorders. The Lancet Diabetes & Endocrinology, 3(4), 286-295. DOI: 10.1016/S2213-8587(14)70225-6