Thyroxine

Indications

Central hypothyroidism is diagnosed by low free T4 combined with low, normal or mildly elevated TSH.

The typical replacement dose is 1.6 mcg/kg/day (e.g., 70 kg man would require ~ 100-125 mcg/day).

In patients with central hypothyroidism, monitor free T4, not TSH.

Patients should be aware that they central hypothyroidism, which is much less common than primary hypothyroidism. They should inform their healthcare providers of that and make sure their thyroid replacement is not inadvertently reduced or stopped based on low TSH levels.
Levothyroxine is ideally taken first thing in the morning, one hour before food, and four hours away from any calcium or iron tablets.
Levothyroxine may worsen untreated adrenal insufficiency. Make sure adrenal insufficiency is ruled out or treated before starting levothyroxine therapy.

Fleseriu M, Hashim IA, Karavitaki N, Melmed S, Murad MH, Salvatori R, Samuels MH. Hormonal Replacement in Hypopituitarism in Adults: An Endocrine Society Clinical Practice Guideline. JCEM. 2016 Nov;101(11):3888-3921.