Drug Update: Hypothyroidism.

Ask 100 endocrinologists what they prescribe for hypothyroidism and you'll generally get the same answer: levothyroxine--synthetic thyroxine, or [T.sub.4].

Experts say synthetic [T.sub.4] therapy most closely mimics the effects of a healthy thyroid gland. Its long half-life keeps the bioavailability of the drug on a much more even keel, compared with the roller-coaster effect of synthetic triiodothyronine, or [T.sub.3], which has a much shorter half-life.

In the last decade, the real advances in managing hypothyroidism haven't been about new therapies but instead about the use of more sensitive thyroid-stimulating hormone (TSH) tests and a better understanding of how to interpret the clinical relevance of their results.

Guidelines from the American Association of Clinical Endocrinologists (AACE) advise against using combinations of thyroid hormones, yet lately endocrinologists have received a steady stream of patients who, armed with information from the Internet, are interested in a combination of [T.sub.3] and [T.sub.4]. In 1999, the results of a controversial study suggested that the combination eased the psychological symptoms of patients with comorbid depression. But at least some experts say that the evidence was too subjective and that it is far too soon to prescribe the combination without further study.

Levothyroxine is the preferred agent to use during pregnancy, but hormone levels must be closely monitored throughout gestation. In nonpregnant patients, follow-up TSH testing is generally done 6 months after a normal hormone level is attained. If that result is normal, patients can be retested after 12 months. During pregnancy, however, follow-up testing should be done every 6 weeks.

Starting dosages of thyroid hormone should be low in the elderly.

 
Drug           Dosage              Cost/Day 
levothyroxine  1.6 [micro]/kg (on  $0.18 * 
 (Levothroid,  average)            (Levothroid); 
 Levoxyl,                          $0.21 * 
 Synthroid,                        (Levoxyl); 
 Unithroid)                        $0.36 * 
                                   (Synthroid); 
                                   $0.37 ** 
                                   (Unithroid) 
                                   (for a 70-kg 
                                   patient) 
liothyronine   25 [micro] b.i.d.   $1.02 * 
 (Cytomel) 
liotrix        120 mg/day          $0.71 * 
 (Thyrolar) 
thyroid,       120-180             $0.09 * 
 Desiccated    mg/day              (120 mg) 
Drug           Comment + 
levothyroxine  Synthetic [T.sub.4]. By far, the 
 (Levothroid,  first-choice hormone replacement 
 Levoxyl,      therapy for patients with 
 Synthroid,    hypothyroidism. AACE guidelines 
 Unithroid)    call for using one brand name 
               formulation and sticking with it 
               throughout a patient's treatment. 
               Starting in August, the Food and 
               Drug Administration will require 
               marketed formulations to have an 
               approved new drug application. The 
               makers of all four listed drugs 
               expect to meet this requirement. 
               Experts stress avoiding switching 
               formulations, if possible. The 
               differences between the various 
               brand name and generic formulations 
               are the dayes and fillers that, 
               theoretically, can affect absorption 
               rates. In addition, some generics 
               have been shown to have a [T.sub.4] 
               contenet that's outside the acceptable 
               range, although the clinical relevance 
               of this is hotly debated. The dosage 
               that achieves normal hormone levels 
               varies substantially, so TSH testing 
               and monitoring are needed. The dosage 
               required to restore a normal TSH range 
               of 0.35-6.2 [micro]U/mL varies depending 
               on a patient's age, weight, cardiac 
               status, and severity and duration of 
               ...