If TSH <3.5 and symptoms of hypoT are present, then treatment is warranted (though personally I would not accept anything less than T4, T3 results to confirm/deny).
TSH secretion is suppressed by T4 and T3; the pituitary releases TSH when T3 & T4 are low, and TSH rises in response to low thyroid function. A TSH over 3.5 is seen in physiologic hypothyroidism. There are very few situations in which both TSH and T4/T3 are low.
Oh, but you may say that this lad has a high prolactin, and prolactin inhibits the secretion of TSH. Nope again, at least not in women:
Prolactin Suppresses GnRH but Not TSH Secretion
G. Page-Wilsona, b, P.C. Smitha, C.K. Welta
aReproductive Endocrine Unit, Department of Medicine, Massachusetts General Hospital, and
bHarvard Medical School, Boston, Mass., USA
Horm Res 2006;65:31-38 (DOI: 10.1159/000090377)
Background/Aims: In animal models, prolactin increases tuberoinfundibular dopamine turnover, which has been demonstrated to suppress both hypothalamic GnRH and pituitary TSH secretion. To test the hypothesis that prolactin suppresses GnRH and TSH secretion in women, as preliminary evidence that a short-feedback dopamine loop also operates in the human, the effect of hyperprolactinemia on GnRH and TSH secretion was examined. Methods: Subjects (n = 6) underwent blood sampling every 10 min in the follicular phase of a control cycle and during a 12-hour recombinant human prolactin (r-hPRL) infusion preceded by 7 days of twice-daily subcutaneous r-hPRL injections. LH and TSH pulse patterns and menstrual cycle parameters were measured. Results: During the 7 days of r-hPRL administration, baseline prolactin increased from 16.0 Â± 3.0 to 101.6 Â± 11.6 Âµg/l, with a further increase to 253.7 Â± 27.7 Âµg/l during the 12-hour infusion. LH pulse frequency decreased (8.7 Â± 1.0 to 6.0 Â± 1.0 pulses/12 h; p < 0.05) with r-hPRL administration, but there were no changes in LH pulse amplitude or mean LH levels. There were also no changes in TSH pulse frequency, mean or peak TSH. The decreased LH pulse frequency did not affect estradiol, inhibin A or B concentrations, or menstrual cycle length. Conclusion: These studies demonstrate that hyperprolactinemia suppresses pulsatile LH secretion but not TSH secretion and suggest that GnRH secretion is sensitive to hyperprolactinemia, but that TSH secretion is not. These data further suggest that the degree of GnRH disruption after 7 days of hyperprolactinemia is insufficient to disrupt menstrual cyclicity.