Dr. Rubin in his Thyroid for Dummies book says this about the lab tests:

"Some of the drugs that can raise the level of TT4 in yoru blood include:
Estrogenic hormones taken for hormone replacement or birth control
Amiodarone, a drug used for the heart
Amphetamines
Methadone

Some clinical states that can raise TT4 levels include
High estrogen states, such as pregnancy
Acute illness, such as AIDS or hepatitis
Acute psychiatric problems

Conversely, some drugs and physical conditions tend to lower the results of a TT4 test (because they depress the amount of thyroxine-binding protein), while not impacting the amount of free thyroxine (FT4). The drugs that have this impact include:
Androgens, male hormones taken to build muscle
Steroids, usually given to reduce inflammation
Nicotinic acid, given to lower harmful blood fats
Asprin in high doses (more than 3,000 milligrams daily)

Physical conditions that can lower TT4 levels include:
Severe chronic illness such as kidney failure or liver failure
Starvation