Q: A friend of mine was told by her doctor that she has thyroid deficiency trouble and says she must take thyroxine or awful things will happen! She looked it up in her medical dictionary and found it was of animal origin. She said she certainly wouldn't take it, especially as it was forever, and thought of going to a homoeopath. She feels perfectly well at present, although she had her blood pressure taken. She is rather on the large size. Then the chemist phoned to say the manufacturer had assured him it was completely synthetic! I wonder if you have heard of this drug and any details of it. M.H., Salisbury.

A: We had a look in the Physician's Desk Reference and the Datasheet Compendium on several thyroxine preparations. Your friend's chemist appears to be right. We note that you both are members of an animal rights organizations. On that front, we can assure you both that you are not offending your principles. Unlike the thryroid hormones class of drug (brand names like Armour Thyroid and Proloid), thyroxines are a synthetic thyroid hormone used to correct thyroid deficiency or to treat goitre. Although it's not clear how the drug works, it somehow alters the processes of cellular chemistry storing energy, thereby speeding up cellular activity. People with a thyroid deficiency are expected to take this drug for life.You mention that your friend is overweight, and you imply that she has high blood pressure. Forest Pharmaceuticals, which manufactures Levothroid, one of the brandname thyroxine preparations, specificially warns that their drug should not be used for the treatment of obesity. In fact, they says, that larger doses can produce serious, "even life threatening" toxic effects, particularly when used with other weight reducing drugs.

Forest and others also caution that this drug could possibly bring on latent coronary artery insufficiency (ie, angina) and diabetes, two conditions for which overweight people are at risk. It can also affect high blood pressure, and overdoses can create hyperthyroidism. Finally, long term use can possibly cause osteoporosis in the spine.

Thyroxines should never be taken with antiasthmatic medications, anti depressants or anticoagulants.

If your friend does decide to go on thyroid medication, she should do so only with supervision by a doctor who will periodically measure thyroid hormone levels in her blood and her blood pressure. But if she feels fine and is not feeling tired and sluggish all the time, two indicators of hypothyroidism, perhaps she should get a second opinion to see if she has a true deficiency requiring permanent use of a drug whose action we don't really understand.