Rationale:
TSH is the best test for assessing thyroid disease. When TSH is within the reference range, additional thyroid tests rarely provide useful information. If TSH is abnormal, further testing should be performed. A normal TSH does not exclude the rare conditions of pituitary failure, hypothalamic dysfunction, or thyroid hormone resistance. If these are suspected, free T4 should also be measured.
In the treatment of primary hypothyroidism with levothyroxine (Levaxin or Euthyrox), therapy is managed based on TSH levels once the patient is stabilized on a consistent dose. In severe cases, fasting free T4 may also be considered. If hypothyroidism is due to pituitary or hypothalamic disease, treatment should be monitored using fasting free T4. Free T3 has no role in diagnosing hypothyroidism or in monitoring hypothyroid patients treated with levothyroxine.
References:
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Garber JR, Cobin RH, Gharib H, Hennessey JV, Klein I, Mechanick JI, Pessah-Pollack R, Singer PA, Woeber KA, American Association of Clinical E, American Thyroid Association Taskforce on Hypothyroidism in A. Clinical practice guidelines for hypothyroidism in adults: cosponsored by the American Association of Clinical Endocrinologists and the American Thyroid Association. Endocr Pract. 2012;18:988-1028.
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McDermott MT. Hypothyroidism. Ann Intern Med. 2020;173(1):ITC1-ITC16.
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Jørgensen og Tazmini (red.). Nasjonal veileder i endokrinologi, april 2022