Can infectious mononucleosis (mono test negative- but antibodies positive- I had it in the past) be the reason for increased TSH? Is T4 necessary?

Unrelated. and T4 / free T4 is necessary if you have abnormal TSH reading, however, clinical context is more important, the doctor who ordered the tests is the best to judge which way to go, good luck .
Mono not related. It is unlikely that former infection with mono has any affect on TSH. TSH is generally elevated if the T4 is low, so it is important to check T4 to determine the decree of hypothyroidism. Ther are several drugs that can influence thyroid and TSH levels, specifically lithium and amiodarone, a heart med. See

Related Questions

Doc said "faintly positive" mono test because he saw I carried the antibody. 4 months ago with same symptoms 2 negative mono tests. Lymphoma?

Monotest. A monotest may be negative in the early stage of infectious mononucleosis and turn positive after a week or two. If your liver enzymes and CBC with differential cell count show no abnormalities, the mono has cleared. Lymphoma isn't often diagnosed by simple blood tests and requires a physical exam, xrays , scan and biopsies for definitive diagnosis. Read more...
Not likely... The diagnosis of lymphoma relies on a constellation of clinical signs and symptoms subsequently confirmed by lymph node biopsy. Even though there is a reported association between mononucleosis and lymphoma, most people with mono never develop lymphoma. If other signs are present like unexplained weight loss, night sweats, fatigue, progressively growing lymph nodes, hematology consult is needed. Read more...