Blood TSH, free T4, free T3 and anti-TPO antibodies should be tested. Patients with an enlarged thyroid and/or a positive anti-TPO antibody test AND a TSH greater than 4.0 mU/mL should be considered to have primary hypothyroidism. Patients without an enlarged thyroid and without a positive anti-TPO antibody test but WITH a TSH greater than 7.5 mU/mL should also be considered to have primary hypothyroidism. Patients with a free T4 of less than 0.9 mg/dL and a TSH less than 1.0 mU/mL are likely to have central hypothyroidism. Patients with symptoms of hypothyroidism but who do not meet these criterion should be watched and retested in six months.
Once hypothyroidism is diagnosed, there are many treatment options, including synthetic L-thyroxine
(T4) preparations (Synthroid, Levoxyl and Unithroid), synthetic L-triiodothyronine (T3) preparations (Cytomel), synthetic T4/T3 combinations (Thyrolar) and dessicated thyroid preparations (Armour, Naturethroid, Bio-Throid, and Westhroid).
All of the L-thyroxine preparations contain the same active ingredient, but contain different fillers and have different quality control. Until recently, Synthroid did not have FDA approval, but now all L-thyroxine preparations have FDA approval. Thyrolar and the dessicated thyroid preparations probably have a higher T3/T4 ratio than desirable and thus, I often give a lower amounts of these preparations supplemented with T4.
Most endocrinologists use L-thyroxine preparations for the initial treatment of all forms of hypothyroidism. Although the use of L-thyroxine (T4) compared to L-triiodothyronine (T3) may be surprising, as T3 is the more bioactive thyroid hormone, T4 is most frequently used. This is because tissues convert T4 to T3 to maintain physiologic levels of the T3. Thus, administration of T4 results in bioavailable T3 and T4. As T4 is more stable than T3, T4 therapy gives even blood levels, while T3 therapy leads to high levels after taking the medicine and low levels before the next dose. Armour thyroid is the least expensive preparation.