Two very common questions I get on my forum are "If I suspect low testosterone, what tests should I get?" and "What testosterone tests will my doctor likely order?"
Unfortunately, there are many physicians out there still new to diagnosing and testing low testosterone and often important items get overlooked. For this reason, until the medical community gets better up to speed as a whole, it is important for men to know the fundamentals of testosterine testing in order to ask important questions.
For this reason I have compiled what I call the 15 Common Tests for Men with Suspected Low Testosterone. These testosterone tests are very important, because they can have such a strong and direct bearing on either a) testosterone levels or b) safe administration of Testosterone Therapy (HRT):
1. Total Testosterone. This testosterone number is the most widely studied and heavily researched number. While it is true that one can debate whether free or bioavailable testosterone is a better indicator, total testosterone gives doctors a very good snapshot as to general testosterone status.
If you've been around my site much, you know that most men begin to experience strong low testosterone symptoms in the 300's (and a few even in the 400's). Furthermore, you hopefully know that in the 300's, a man's risk factors for many serious medical conditions (diabetes, osteoporosis, Metabolic Syndrome, erectile dysfunction, etc.) begin to increase signficantly, something I document in my book Low Testosterone By The Numbers. (NOTE: Some men seem to do fine and are symptom-free in the 300's and 400's, however.) Total testosterone, and all the tests on this page for that matter, are generally pulled through a blood draw and should be done as early in the morning as possible (since a man's testosterone falls throughout the day).
So what will your doctor be looking for? From what I have seen, most knowledgeable doctors will treat a man (with no risky medical conditions) if he is in the 300's and classic low testosterone symtpoms. Some doctors simply will not treat any man in the 300's and often the upper 200's. Many of the laboratory reference ranges have their minimum for total testosterone still sitting in the upper 200's and so doctors will classify any man above around 260 as "normal". For example, one of the biggest labs that physicians use (in the U.S.) is Quest Diagnostics and Quests "normal" testosterone ranges for adult males, ages 18-69 is listed as 250-1100 ng/dl.  Other similar ranges have been referenced on the forum as well:
NOTE: Non-U.S. readers should use a conversion factor of 29.4 to convert to nmol/l.
Notice that all of these reference ranges are between 241-286 ng/dl. Let's say that your lab results show up with the 241 number and your actual total testosterone is 242. Many physicians will simply say, "Sorry, fella, but you are clearly normal. You are clearly within the normal range." Meanwhile, you could be limping through life with erectile dysfunction, horrendous memory and concentration, a non-existent libido and a complete loss of morning erections. But, by golly, that doc is absolutely certain that your issues have NOTHING to do with low testosterone because of that lab result and the range listed right beside it.
Fortunately, many doctors are now realizing that the 200's are abyssmally low numbers for most men. Furthermore, the biggest lab, LabCorp, has now bumped up the lower end of their total testosterone range for males over 18 years old to 348-1197 ng/dl.  Of course, the lower end of this range for normal testosterone, 348 ng/dl, is higher than past ranges. Even better, I have seen this figure quote by several on the Peak Testosterone Forum.  Essentially, LabCorp is arguing that our current definitions of hypogonadism are much too low and, perhaps, doctors will listen.
2. Free Testosterone. Free testosterone, which can be thought of as the testosterone that can actually do work on your tissues and receptors, runs about 2% of total testosterone. An acceptable range for testosterone is often give as 1.5-2.5%. To read in more detail, see this page on Free Testosterone and SHBG.
3. Leutinizing Hormone (LH). Leutinizing Hormone is secreted from the pituitary at night in pulses and stimulates the Leydig cells to produce testosterone. Thus, LH is responsible for (the great majority) of testosterone in men.
Plasma levels of LH are useful primarily for diagnostic purposes, because there are two "categories" of low testosterone: primary and secondary hypogonadism. In the case of primary hypogonadism, usually LH and FSH are overly high and this indicates that the problem is in the testes. Basically, the pituitary tries to compensate for the testes' lack of production by trying to pump out more LH in order to normalize testosterone production. LH just keeps climbing and climbing with no significant change in testosterone.
In the case of secondary hypogonadism, both LH and FSH are generally low or low normal and, in this case, the problem is somewhere rooted in the pituitary or the hypothalamus rather than in the testes themselves. Thus, with secondary hypogonadism the problem is not in the testes but in the brain's signaling to the testes.
Knowing whether you are primary or secondary can help a doctor know best how to treat you, or, at least, a knowledgeable one Some treatment methods, such as Clomid, are designed to work on men with secondary hypogonadism as a root issue.
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4. Follicle Stimulating Hormone (FSH. FSH is also produced by the pituaitary and, coupled with testosterone, is responsible for sperm production. Doctors will pull this number primarily for fertility purposes.
5. Estradiol (E2). There are actually several estrogens in the blood stream of both men and women. Estradiol is the most well-known and "potent" estrogen and is responsible for most of its properties, both good and bad for us males.
Estradiol is one of the most important, and often ignored, hormones that should be pulled in men who are experiencing sexual or erectile dysfunction issues. The reason is that estradiol has a relatively tight range that men should, ideally, fall within. If estrogen goes too high (in men), it can cause many of the same issues associated with low testosterone: erectile dysfunction, low libido, fatigue, etc. Furthermore, it is a) fat-promoting, b) cancer promoting (in the prostate) and c) potentially fuels gynocomastia ("man-boobs").
Overly high estradiol can cause issues, but, more often that not, low estradiol is the culprit in men with lowered libido and erectile strength. It is very important that you get the right estradiol test, which is an LC-MS/MS test designed for the low levels that men have. Low estradiol can lead to bone less and eventually osteoporosis, but an important new study shows that low estradiol causes virtually the exact same symptoms associated with low testosterone.
6. Zinc Status. Every doctor should, in my opinion, test for zinc status. There are several tests that are applicable including the zinc taste test and zinc plasma levels. And the reason is that, if you are low in zinc, restoring proper zinc levels can double or more your testosterone levels. See this page on Zinc Deficiencies and Low Testosterone for more details.
7. Vitamin D. One of the most important low testosterone tests is actually Vitamin D, due to the fact that Vitamin D deficiencies are so widespread. The standard for Vitamin D monitoring is called "25-hydroxy" test and can be obtained from a simple blood draw. Therefore, it can be pulled at the same time that you have your testosterone tested. Most labs and physicians now consider 30 ng/ml to be the threshold for a deficiency. However, many experts recommend that Vitamin D be maintained somewhat above that level.
What does your Vitamin D reading have to do with testosterone? It turns out Vitamin D is actually correlated with testosterone and appears to be partially causative. In other words, keeping your Vitamin D levels up to healthy levels may give you a boost in testosterone. (Some experts even consider Vitamin D much more a hormone than a vitamin.) For more information, see my link on The Benefits of Vitamin D.
8. Prolactin. If this hormone goes too high, it drive down testosterone levels by negatively impacting dopamine. The net effect is usually significantly lowered libido and sexual function. I cover this in more detail in my link on Prolactin and Prolactinomas.
9. Thyroid Stimulating Horomone (TSH). If this is high, then you may have hypothyroidism. TSH actually comes from the pituitary and not the the thyroid gland. In general, when the pituitary senses that your thyroid hormones are falling too low, it pumps out more TSH in order to compensate, similar to elevated Leutinizing Hormone (LH) in men with primary hypogonadism. However, TSH is just the beginning and free T3, free T4 and reverse T3 should also be pulled at a minimum.
What does this have to do with testosterone? Well, researchers have found that hypothyroidism is associated lower testosterone levels and it is no wonder since it usually comes with fatigue, weight gain and a slowed metabolism and libido, i.e. symptoms that are very similar to those of hypogonadism. Furthermore, correcting hypothyoridism can lead to a nice boost in testosterone often, something I discuss in my page on Testosterone and the Thyroid.
10. T4 and T3. These two thyroid hormones, which are triggered by TSH coming from the pituitary, can be low as well. Somewhat like testosterone, these can come as total T4 and/or T3 and free T4 and/or T3. When these are below the reference ranges, you will usually find that your physician will consider you hypothyroid.
11. PSA. Physicians will generally monitor your prostate before administering TRT, HCG Monotherapy or Clomid over concern regarding prostate enlargement (BPH) and prostate cancer as a side effect. Several large studies have hsown these not to be a concern, but, as I always say, you have to discuss with your doctor and do your own research. Regardless, it is prudent to pull your PSA before TRT in my opinion. The PSA has been under sharp criticism in the last ten years, because it does not just show an increase in cancer but also an increase in prostate size and inflammation. In fact, the latter is the most common reason for a significant rise in PSA. (It happened to me.)
12. Red Blood Cell Count/Hematocrit/Hemoglobin. Testosterone actually governs male red blood cell counts in your blood and this is the reason that we have more red blood cells in our plasma than our wives and girlfriends. A good physicians will always monitor your RBC count, as it is called, to make sure that you are neither too high or too low. However, the key is that, if you are anemic from low testosterone, then something probably needs to happen.
Again, men with low testosterone often see their RBCs fall significantly and can even end up anemic. This is yet another reason that men can experience fatigue with hypogonadal testosterone levels.
There is another reason to get RBC Counts/Hematocrit/Hemoglobin: if a man is on the high side of these numbers before TRT, he may struggle while on TRT trying to keep his levels in check. You cannot let your levels go too high or it can increase the risk of stroke or an MI. (Some of the Tour De France riders have almost died from this condition due to doping).  One of our forum posters actually found himself in this condition, for example, even though his most recent testosterone reads were 290 and 301 ng/dl. 
So what are acceptable RBC counts? Different labs have a little different ranges, but the range given as normal is generally around 4.3-5.7 million cells/mcl. Notice, however, that this forum poster's range was 4.1-5.6 and thus differently slightly.  I have seen higher levels as well, say, at 4.7-6.1.
Also, some men may wonder why testosterone increases red blood cell counts. The reason actually ties into a kidney hormone called erythropoietin which induces red blood cell production in the stem cells of the bones. Testosterone actually increases erythropoietin production and, therefore, actually causes an increase in red blood cells due to its downstream influences.
13. SHBG. This is the binding protein that attaches itself to a little over half of our testosterone and effectively takes it out of commission. If you are low SHBG, you may struggle with TRT and require a little different protocols. This is also possible with high SHBG as well. Low SHBG men may also be insulin resistant, something I discuss in my page on Low SHBG.
14. Other Hormones. If you end up on TRT, just trust me: you'll be glad that you pulled your DHEA-S, progesterone and cortisol and established some kind of a baseline. There is one school of thought out there that TRT can slow down these hormones over time, so pull these while you can to see if you have changed over time. And, by the way, all of these can affect sexual function and energy levels.
15. Liver Function. This is an important test for low testosterone men,. because testosterone is metabolized in the liver. A good physician at a minimum will check to make sure that you do not have any existing liver disease or disorders. In addition, some of the injectables (cypionate, enanthiate, etc.) actually have to be broken down into testosterone. The topicals (Androgel, Testime, etc.) have a good liver side effect profile but should be checked regardless from time to time as occasional issues do occur.
1) Quest Diagnostics, Table 1, Testosterone Reference Ranges in Adults, http://www.questdiagnostics.com/hcp/ intguide/EndoMetab/Gen_Misc/Testosterone/Table%201.pdf
5) Labcorp, Technical Review, "Testosterone: Aiding in the diagnosis of androgen dysfunction and hypogonadism"
8) Dtsch Arztebl Int, Jan 2008, 105(4):62 68, "Congenital and Acquired Polycythemias"